I dreamed this cake up one day while driving. I had Cristina Tosi's approach to cake on my mind (Sarah was trying out her classic birthday cake -- I hope she'll share her thoughts soon!). What\ piqued my interest was the textural contrast Tosi goes for with the cake crumbs. It sounded really, really tasty. I began thinking where you could go with that. "Oooh! Shortbread crumbs!" It didn't take long for a giant twix cake to sound tremendously delicious.

I quickly decided on a chocolate cake base, especially if I was going to use shortbread crumbs in between the layers. Chocolate buttercream was a must too. What stumped me was whether to do plain shortbread crumbs in between the layers with a caramel sauce drizzle or make un-dipped twix bars and break those in. It was a bit of a dilemma. A video chat with Sarah helped decide the matter, and she offered fantastic decorating tips too. Win win!

Did the cake live up to my dream? Yes. A very happy yes. The shortbread crumbs are what made it for me, providing a delicious, buttery, surprising crunch against the deep, soft cake. Each component is scrumptious on its own (I'd make them each again on their own or for different recipes), but altogether they make a fantastic sweet & salty combo in this cake. And while the whole thing uses quite a few eggs, it's worth it. (And take a look here to decide how to use up those pesky extra yolks.) This is a party cake I'll make again without any tweaking. 

Twix cake

Serves: A great many. At least 18 thin pieces, and a Wusthof serrated knife comes in very handy.

Ingredients:

[For the Shortbread Crumbs:] I used the base from Sarah's millionaire bars.*

• 1 stick (4 oz) of cold butter, cut into small cubes

• 1 cup of all-purpose flour

• 1/3 cup of white sugar

• 1/4 teaspoon of salt

[For the Cake:]

• 3 sticks butter (12oz)

• 1 1/2 cups brewed black coffee, or black tea such as Assam (12oz)

• I cup dutch cocoa powder (3oz)*

• 6 ounces finely chopped dark chocolate*

• 2 cups light brown sugar (16oz)*

• 1 tbs vanilla extract (1/2oz)

• 1 tsp kosher salt (4g); for table salt, use half as much by volume or the same weight

• 6 large eggs, straight from the fridge (about 10 1/2 ounces)

• 3 large egg yolks, straight from the fridge (about 1 1/2 ounces)

• 2 cups all-purpose flour (9oz)

• 1 tbs baking soda

[For the Caramel:]

• 2 cups granulated sugar

• 12 tbs unsalted butter, cut into tbsp-size chunks

• 1 cup heavy cream

• 1 tsp fine sea salt, plus more to taste*

[For the Frosting:]

• 8oz semisweet chocolate, chopped*

• 2oz unsweetened chocolate, chopped*

• 1 lb unsalted butter, room temperature

• 5 egg whites (roughly 150g)

• 1 cup granulated sugar

Directions:

[For the Shortbread Crumbs:]

1. Preheat the oven to 350 degrees. Line a 9x9-inch pan with parchment paper. Set aside.

2. In a mixing bowl whisk together flour, sugar and salt. And the butter. With a pastry cutter, blend the mixture until it resembles coarse crumbs. (Alternatively, you can use a food processor. Place ingredients in the processor and pulse.) Pour the crumb mixture into the prepared pan and use your fingers to spread it out and press it down. You can use the bottom of a measuring cup to press down and create a level crust.

3. Bake in the preheated oven until golden, about 20 minutes. Go for a really nice golden brown -- you'll get more crunch in the cake! Once fully baked, set aside to cool.

4. Once fully cooled, remove shortbread from pan and break into crumbs. You should have about 2 1/4 cups of crumbs.

5. Note: If not using immediately, store in an airtight container to prevent getting soggy.

[For the Cake:]

1. Preheat oven to 350. Butter two 9x13in pans and line with parchment.*

2. Combine butter and coffee (or tea) in a 5qt pot over low heat. Once melted, remove from heat. Mix in cocoa and chocolate, then add brown sugar, vanilla, and salt. Mix in eggs and yolks. Sift in flour and baking soda, and whisk thoroughly to combine.

3. Divide batter equally between prepared cake pans. (If you don't have two pans, the remaining batter can stay at room temp for up to 90 minutes, though it won't rise as much.) Bake until cakes are firm but still soft enough for your finger to leave a print on the crust (or until a toothpick comes out with some crumbs), about 20 min.

4. Cool cakes in their pans for 1 hour, then run a knife around the edges to loosen. Turn over onto a wire rack, remove parchment, then return cakes right side up.

[For the Caramel:]

1. Put granulated sugar in a large nonstick frying pan and spread evenly. Heat over medium-high heat, stirring constantly. Sugar will start to form clumps but eventually melt and turn into a dark, amber-colored liquid. This takes about 6 min.

2. Once the sugar is liquified, reduce heat to medium-low. Add butter and stir until fully melted and mixed. Stirring constantly, drizzle in cream.

3. Boil mixture for 1 min, stirring, then remove from heat.

4. Add sea salt, then scrape into a bowl and let cool completely at room temp. Once cool enough to taste, adjust salt to liking.

5. Note: This makes a lot of caramel sauce -- more than you'll need. But, I recommend having it while serving the cake to drizzle over the sliced pieces. Room temp makes for easier drizzling, but even from the fridge, this sauce remains fairly soft. Whatever remains after that makes for an excellent ice cream topping, crepe filling (plus coconut -- yum!), oatmeal add in, etc...

[For the Frosting:]

1. Heat 1 in of water in a small saucepan. Place chocolate in a medium heatproof bowl and place over the simmering water, making sure the bottom of the bowl does not touch the water. Melt chocolate, stirring until smooth. Set aside, but keep the saucepan at a simmer.

2. Place egg whites and sugar in a bowl of a stand mixer. Set bowl over simmering saucepan. Whisk gently until mixture reaches 140°F. Remove from heat.

3. With the whisk attachment, beat egg white mixture on level 4 until stiff. Turn down to level 3 and whip until bottom of the bowl is cool to the touch (this can take a bit).

4. Change to a paddle attachment and gradually add soft butter one tbs at a time, mixing well after each addition to emulsify. Once all the butter has been added, beat until it reaches desired consistency. Fold in cooled chocolate until well mixed.

To Assemble:

1. Cut each 9x13inch cake in half length-wise. (You want to end up with four 4.5x13in layers.) Take one layer of cake and top with 1/2 cup of frosting. Take one cup of shortbread crumbs and sprinkle evenly over the entire layer. Gently press crumbs into the cake to keep intact. Drizzle with 1/2 cup caramel.

2. Take a second cake layer and place on top. Frost with 1 cup of frosting.

3. Take a third cake layer and place on top. Frost with 1/2 cup of frosting, followed by 1 cup crumbs, then 1/2 cup caramel.

4. Take the final cake layer and place on top. Frost with remaining frosting using an offset spatula. Try to get the edges as sharp as possible -- a little tricky! Store in fridge until ready to decorate and serve.

5. When ready to decorate and serve, remove cake from fridge. Using about 1 cup of caramel, pour over the top of the cake. Using an offset spatula, gently spread the caramel and nudge it to the sides of the cake. The more you nudge it to the edge, the more it will drip down. Get it just over the edge and gravity will do the rest.

6. Sprinkle the remaining shortbread crumbs down the middle of the cake and serve. The longer the cake sits once topped with caramel, the more the caramel will drip down the sides and puddle at the bottom. Still delicious and pretty, but not quite as spectacular.

7. Note: this cake keeps well for 1-2 days, but any longer than that, the shortbread crumbs begin loosing their crunch. To make ahead, make components separately and assemble on the day you plan to serve.

Notes:

• If you think I really ought to have gone the un-dipped twix route, just make the millionaire bars sans the chocolate topping. Chop up and add in between the layers instead of the shortbread crumbs & caramel sauce. Just make sure to save some for the top as well! (If you still want the caramel drip on top, 1/4 a recipe should be enough, though make 1/2 if you'd like leftovers :)

• Dutch cocoa is called for, but regular would be fine. The cake likely won't be quite as dark. Hershey's special dark is a good, less expensive alternative.

• I only had chocolate chips on hand. No ill effect!

• I realized I only had one cup of brown sugar too late. Whoops. Luckily enough, it is way too easy to make your own.

• DON'T use regular table salt -- it will be much too salty & sharp.

• Again, only semisweet chocolate chips on hand. I've made this frosting many, many times and prefer it as written. The milk chocolate profile may be more authentically twix bar though! So, go with your preference.

• I have a large 18x12in rectangle pan which made 4.5x12in layers. Two 9x13in pans are more likely to be in a standard kitchen, and just as good. The slightly thinner layers may even be an improvement. I liked the quasi-candy bar shape, but this could also make three 8in or 9in cake layers.

Adapted from these recipes: Devil's Food Cake, Chocolate Thumbprint Cookies, Chocolate Espresso Fudge Cake Buttercream, and Millionaire Bars.