Prep time: 30 minutes
Total time: 13 hours
for the cake:
12 tablespoons unsalted butter, plus more for greasing
4 1/2 ounces milk chocolate, roughly chopped
14 ounces dark chocolate, roughly chopped
5 large eggs, lightly beaten
3/4cup warm coffee
3/4 cup cocoa powder
1 3/4 cups all-purpose flour
1/4 teaspoon baking powder
1/4 teaspoon baking soda
2 1/4 cups light brown sugar
1/2 teaspoon vanilla extract
for the wafer crust:
3 cups crushed chocolate wafer cookies
1/4 cup light brown sugar
pinch kosher salt
1/2 cup unsalted butter, melted
for the mousse:
1/4 cup whole milk
1/2 vanilla bean, seeds scraped
2 sheets gelatin
1 1/2 cups melted milk chocolate
1 egg yolk
3 egg whites
1/4 cup sugar
1/2 cup heavy cream
1. Make the cake: Heat the oven to 325°F|160°C. Grease a 13-inch by 18-inch sheet tray with butter and set aside.
2. Pour enough water into a 4-quart saucepan that it reaches a depth of 1-inch. Bring to a boil and reduce the heat to maintain a simmer. Combine the butter and chocolates in a medium bowl and set the bowl over the saucepan. Cook, stirring, until melted and smooth, about 5 minutes. Remove from heat and set aside.
3. Whisk the eggs together in a large bowl. In a separate bowl, combine the coffee with the cocoa powder and whisk until smooth, then stir into the eggs. In a separate bowl, combine the flour with the baking powder and baking soda. Add the sugar and vanilla to the egg mixture and whisk to combine. Stir in the chocolate mixture and fold in the flour mixture.
4. Spread the batter evenly into the prepared pan and bake until fluffy and cooked through, about 20 minutes. Cool slightly, then cut 2 8-inch circles out of the cake.
5. Make the crust: In a medium bowl, mix 2 cups of the cookies, sugar, and salt with the melted butter. Press into the bottom of a 9-inch high sided springform pan and bake until slightly firm, about 10 minutes.
6. Make the mousse: Heat the milk and the vanilla seeds in a small saucepan over medium. Meanwhile, bloom the gelatin in water until soft, about 10 minutes. Squeeze out the liquid and add it to the warm milk, stirring to dissolve.
7. Pour enough water into a 4-quart saucepan that it reaches a depth of 1-inch. Bring to a boil and reduce the heat to maintain a simmer. Add the chocolate in a medium bowl and set the bowl over the saucepan. Cook, stirring, until melted and smooth, about 5 minutes. Remove from heat and stir in the yolks, then whisk in the milk mixture until smooth.
8. Meanwhile, in a large bowl, whip the egg whites until stiff peaks form, gradually adding in the sugar. Fold the whites into the chocolate mixture 1/3 at a time until incorporated.
9. Whip the cream until soft peaks form, then fold into the chocolate mixture 1/3 at a time until incorporated.
10. Assemble your cake: Layer about 1 cup of the mousse onto the crust and top with one circle of cake. Layer another 1 cup of the mousse and another layer of cake, then top with the more mousse. You might have some leftover, and that's ok. You have cake scraps. Make it into a trifle or something and eat it for dinner. Freeze the cake for at least 12 hours.
11. To serve, thaw slightly in the fridge and garnish with the remaining cookie crumbles.