Special equipment: three 9-inch cake pans; a candy thermometer
For the cake: Preheat the oven to 350 degrees F. Butter three 9-inch cake pans and dust with cocoa powder, shaking off the excess.
Mix the chocolate into the hot coffee in a medium bowl and stir to melt. Set aside to cool completely.
In another bowl, sift together the flour, cocoa powder, baking soda and baking powder. Set aside.
Combine the sugar, oil, salt and vanilla in the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed, 3 to 5 minutes. (This batter is fluid and tends to get lumps; mix carefully with the paddle, or even use the whisk attachment on low speed to avoid lumps.) With the mixer on low speed, add the eggs, one at a time, mixing well after each addition. Scrape the bowl and continue mixing on medium-low speed until fully incorporated, 2 to 3 minutes. (Don't worry about overmixing at this stage; you need to make sure the batter base is fully mixed.)
Stir the buttermilk into the coffee mixture. In alternating batches, add the flour and buttermilk mixtures to the batter base. Mix until just combined, being careful not to overmix. The batter should be thick but fluid. Stir or mix by hand to get rid of any lumps; strain if you feel compelled.
Divide the batter among the prepared cake pans. Bake until a toothpick comes out clean or with just a few crumbs, 20 to 30 minutes. Let cool completely.
For the toffee: Line a baking sheet with parchment paper.
Combine the butter, sugar and salt in a heavy-bottom saucepan fitted with a candy thermometer. Heat over medium-high heat to 300 degrees F (the hard crack stage), 7 to 10 minutes. Remove from the heat and stir in the vanilla extract. Pour over the prepared baking sheet and let cool completely. Break into small pieces, then process into a powder using a food processor or coffee grinder.
For the buttercream: Combine 339 grams of the sugar, the water, corn syrup and salt in a heavy saucepan fitted with a candy thermometer. Heat over medium-low heat until the sugar dissolves. Increase the heat to medium, being careful not to let the flames go up the sides of the saucepan. The target temperature for the syrup is 250 degrees F.
Meanwhile, combine the egg whites and cream of tartar in the bowl of a stand mixer fitted with the whisk attachment. When the syrup has reached 230 degrees F, run the mixer on medium speed until the egg whites get frothy. Sprinkle the remaining 30 grams sugar into the egg whites and continue to whip on medium speed. (The egg whites should increases in size and form soft peaks by the time syrup reaches 250 degrees F.) Once the syrup reaches 250 degrees F, increase the mixer to high speed and carefully pour the hot syrup down the side of the bowl, being sure not to hit the whip. Whip the mixture until it has cooled to room temperature, 5 to 10 minutes.
Replace the whisk with the paddle attachment. Gradually beat in the butter, a few cubes at a time, on medium-low speed. Mix until the buttercream comes together, then mix in the vanilla bean paste.
Mix 3 to 4 ounces of the powdered toffee into 1 cup of the buttercream and set aside.
For the chocolate feuilletine crunch: Line a baking sheet with parchment paper.
Melt the dark chocolate and milk chocolate in a double boiler. Remove from the heat and mix in the feuilletine. Spread on the prepared baking sheet and let cool. Crumble into bite-size pieces. Set aside.
To assemble the cake: Set one cake layer on a serving plate. Spread with a thin, even layer (about 1/4 inch) of the buttercream, then spread with a thin, even layer (1/8 to 1/4 inch) of the toffee-buttercream mixture. Top with feuilletine crunch. Repeat with the remaining layers. Cover the cake with the remaining buttercream.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.