"Magnifique" is really the only appropriate word to describe this masterpiece of chocolate and caramel. However, my straight-talking pipe fitting buddy did a nice job when he summed it thus: "Dude, that sh*t is so good!"
1 3/4 cups all-purpose flour
4 1/2 tbsp cornstarch
1 tsp baking powder
1/2 tsp baking soda
1 1/4 cups Callebaut or other fine cocoa powder
1 tsp kosher salt
1 cup nnsalted butter, room temperature
2 3/4 cups sugar
5 large eggs
1 1/4 cups buttermilk
24 oz. Callebaut or other fine bittersweet chocolate
3 cups heavy cream
1/3 cup heavy cream
1/2 tsp fine vanilla
1 1/4 cups sugar
1/4 cup water
1/4 tsp salt
2 tbsp light corn syrup
1 tsp fresh lemon juice
4 tbsp unsalted butter
First, make the caramel by pouring the cream into a small, heavy saucepan and stirring in the vanilla. Place over medium-high heat and bring to just under a boil, stirring occasionally. Reduce the heat to low to keep the cream warm.
In a medium, heavy saucepan, combine the sugar, water, salt, and corn syrup. Bring to a boil over medium heat, stirring to dissolve the sugar. Then cook, without stirring, until the mixture is light amber colored, 5 to 8 minutes. It may not color evenly. That is fine. Still don't stir it. Remove from the heat.
The mixture will continue to cook off the heat and become darker, so make sure to have your cream close by. Carefully and slowly add the cream to the sugar syrup. The mixture will boil vigorously at first. Let the mixture simmer down, and then whisk until smooth. Add the lemon juice. Let cool for about 5 minutes.
Cut the butter into 1-inch chunks and add them to the caramel one at a time, whisking constantly after each addition. Then whisk the caramel periodically as it continues to cool. Set aside until needed.
Preheat the oven to 350°F. Butter and lightly flour the sides of two 9-inch cake pans, knocking out the excess flour. Line the bottom of each pan with parchment paper cut to fit exactly.
To make the cake layers, sift together the flour, cornstarch, baking powder, baking soda, cocoa powder, and salt into a bowl and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until light and creamy. Slowly add the sugar and continue to beat on the same speed until light in color and fluffy. Add the eggs one at a time, mixing well after each addition until incorporated before adding the next egg. Stop the mixer and scrape down the sides of the bowl with the rubber spatula. With the mixer on low speed, add the flour mixture in 3 equal batches alternately with the buttermilk in 2 batches, beginning and ending with the flour mixture. Stop the mixer, scrape down the sides of the bowl, and then mix again for another few seconds.
Divide the cake batter evenly between the prepared cake pans. Bake until a cake tester inserted in the center comes out clean, about 40-45 minutes. Let the cakes cool completely in the pans on a wire rack.
When the cakes are cool, turn them out by inverting the pans, and then turn the cakes upright. Using a serrated knife, slice off the domed portion from the top of each cake to make the tops flat. Reserve the slices for making the crumbs.
To make the crumbs, preheat the oven to 250°F. Line a baking sheet with parchment paper or a nonstick liner. Break up the cake slices and spread on the lined baking sheet. Place in the oven and toast until completely dry, about 1 hour. Let cool completely, then transfer to a food processor or blender and process until finely ground. Sift the crumbs through a medium-mesh sieve. Don't use a sieve with fine mesh or the crumbs won't pass through. Set aside.
To make the ganache, place the chocolate in a heatproof bowl. Bring the cream to just under a boil in a small saucepan. Pour the cream over the chocolate. Let the mixture sit for a few minutes without stirring until the chocolate is partially melted, and then stir with a rubber spatula until smooth and shiny.
To assemble the cake, split each cake into 2 layers to make 4 layers in all. Transfer 1 layer to a serving plate. Using an offset spatula, spread 1/4 cup of the caramel evenly over the cake layer.
Spread a thin layer of ganache (about 1/4 inch thick) over the caramel. Top with a second cake layer, and again spread with 1/4 cup caramel and then a thin layer of ganache. Repeat with the third cake layer, the remaining 1/4 cup, and a thin layer of ganache. Top with the fourth cake layer. Refrigerate the cake until the center seems firm, 1 hour. Cover the remaining ganache with plastic wrap and leave at room temperature for finishing the cake.
Remove the cake from the refrigerator. Using the rest of the ganache, frost the top and sides of the cake with the offset spatula. The cake must be evenly frosted and the ganache must be soft enough for the crumbs to adhere. If the ganache has hardened, use a kitchen torch to soften it slightly, or put the whole cake into a 400°F oven for 40 seconds or so, just until the chocolate looks shiny. Sprinkle the crumbs evenly over the top of the cake, then slightly tilt and turn the cake so that they spill over the sides, adhering to them as they fall.
Serve the cake at room temperature. To store, cover and keep in a cool place for up to 4 days. It is not necessary to keep this cake in the refrigerator.