Step 1Heat the oven to 350 degrees. Place the racks in the upper and lower thirds of the oven. Grease 3 (8-inch) round cake pans, and line the bottom of each with a round of parchment paper.
Step 2In a large bowl, whisk together the flour, cocoa powder, baking soda and salt.
Step 3In a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, cream together the butter and sugar until light and fluffy, 3 to 5 minutes. Add the eggs, one at a time, mixing until each is incorporated. Add the vanilla and mix to combine. Beginning and ending with the dry ingredients, add the flour mixture and coffee separately, a little at a time, and blending well after each addition.
Step 4Divide the batter among the prepared cake pans and place in the oven. Bake the cake layers until each is puffed and set, and a skewer comes out clean, 30 to 40 minutes, rotating halfway through for even baking.
Step 5Cool the cake layers in their pans on a rack. If not using right away, unmold and cover the cooled cake layers tightly with plastic wrap; the cakes will keep, covered, for one day at room temperature and up to 2 months frozen (thaw before using).
Step 6To make the crumble garnish, trim each cake layer so it is about 3/4-inch thick. Crumble the trimmings onto a parchment-lined rimmed baking sheet and bake for 30 minutes at 350 degrees. Grind the trimmings in a food processor to attain a fine, even crumb. This makes about 6 cups crumble garnish, more than is needed for the remainder of the recipe.
Step 1Finely chop the chocolate and place in a large bowl. Set a strainer over the chocolate in the bowl and set aside. Meanwhile, in a medium bowl, whisk together the egg yolks and sugar.
Step 2In a heavy-bottom saucepan, bring the milk and cream just to a simmer. Slowly add about 1/3 of the hot cream to the egg mixture, whisking to temper the eggs, then add the egg mixture to the saucepan. Cook over low heat, stirring constantly, until the mixture thickens and a thermometer inserted reaches 180 to 183 degrees. Immediately remove from heat and pour the mixture into the strainer over the chocolate in the bowl, discarding any curdled bits.
Step 3Stir the custard in with the chocolate until the chocolate is melted and incorporated with the custard. If the custard is lumpy, process with an immersion blender until smooth. Place the chocolate custard over an ice bath and stir until the custard is cooled. Cover the surface of the custard with plastic wrap and refrigerate for one day before assembling the cake to give the custard time to set up. This makes a generous 2 cups crémeux, more than is needed for the remainder of the recipe; the crémeux will keep, covered and refrigerated, up to 3 days.
Step 1In the bowl of a stand mixer or a large bowl set over a pot of simmering water, whisk the egg whites and sugar until the sugar dissolves and the mixture is hot to the touch (around 160 degrees). Take the egg mixture off the heat and beat it in a stand mixer fitted with the whisk attachment or using a hand mixer, until stiff peaks form and the eggs are no longer hot.
Step 2With the mixer running, slowly add the softened butter, beating until each addition is incorporated. Once all of the butter has been added, and the buttercream is light and fluffy, slowly stream in the espresso. Continue mixing until the espresso is incorporated. This makes a generous quart of buttercream, more than is needed for the remainder of the recipe. The buttercream is best made the day of assembly, but can be covered and refrigerated up to 3 days before using (bring the buttercream back to room temperature and whip once more before using).
Step 1Chop the chocolate and melt it in a bowl set over a pan of simmering water, stirring until the chocolate is smooth; the chocolate can also be melted using a microwave. Set aside.
Step 2In a small heavy-bottom saucepan, combine the cream and honey over high heat. When the mixture comes to a boil, remove from heat. Pour 1/3 of the cream over the chocolate, stirring to combine. Add the rest of the cream in two stages, stirring each until combined. Add the diced cold butter, stirring until the butter is melted and incorporated. If there are any leftover bits of butter, use an immersion blender to blend and smooth out the mixture. This makes about 1 1/3 cups glaze. If not using right away, cover and refrigerate up to 3 days. Warm the glaze to pourable consistency before using.
Step 1To assemble the cake, brush the first cake round with the simple syrup to moisten. Spoon a generous 1/2 cup of the crémeux over the cake and spread it in an even layer to the edge of the round. Spoon about 3/4 cup buttercream on top of the cake, over the crémeux, and spread the buttercream in an even layer to the edge of the round.
Step 2Place the second cake round over the first and repeat, first with the simple syrup, then the crémeux and buttercream. Place the third cake round over the top. Moisten using the remaining simple syrup, then spoon the remaining crémeux over the top, spreading it over the top and sides of the cake in a thin even layer. Refrigerate the cake for at least 30 minutes to chill.
Step 3Place the chilled cake on a rack set in a rimmed baking sheet. Pour over the glaze, making sure it covers the top and sides of the cake. Press the crumbles to cover the sides of the cake. Refrigerate the cake to store, and bring to room temperature before serving.