FOR THE CAKE :
8 tablespoons (I stick) unsalted butter, softened
¾ cup cocoa powder, preferably Dutch-process
¼ cup vegetable shortening
2 cups sugar
2 teaspoons pure vanilla extract
1 teaspoon baking powder
1 teaspoon baking soda
½ teaspoon salt
2¼ cups cake flour
1 cup milk
FOR THE CUSTARD:
3 cups water
2½ cups sugar
1 tablespoon corn syrup
Scant 2/3 cup cornstarch
1½ cups cocoa powder, preferably Dutch process
6 tablespoons unsalted butter, cut into pieces
½ teaspoon pure vanilla extract
Heat the oven to 375º F. Butter and flour two 9-inch round cake pans. Cut 2 circles of parchment paper or wax paper to fit the bottoms of the pans, then press them in.
MAKE THE CAKE:
Cream the butter and shortening together in a mixer fitted with a whisk attachment (or using a hand mixer). Add the sugar and mix until light and fluffy. Add the eggs one at a time, mixing after each addition. With the mixer running at low speed, add the vanilla, cocoa, baking powder, baking soda, and salt and mix. With the mixer running at low speed, add about a third of the cake flour, then about a third of the milk, and mix. Repeat with the remaining cake flour and milk and mix.
Pour into the pans and bake until dry and springy to the touch and a tester inserted into the center comes out clean (a few crumbs are okay), 30 to 35 minutes. Let cool in the pans for 15 minutes, then turn out onto wire racks and let cool completely, to room temperature.
Using a long serrated knife, cut the cake layers horizontally in half. Reserve 3 halves for the finished cake and put the remaining half in a food processor, breaking it up with your hands. Pulse into fine crumbs.
MEANWHILE, MAKE THE CUSTARD
Pour 2½ cups of the water, the sugar, corn syrup, and cocoa powder into a large saucepan and bring to a boil over medium high heat, whisking occasionally. Dissolve the cornstarch in 1/2 cup of water. Whisk into the cocoa mixture in the saucepan and return the mixture to a boil, whisking constantly. Cook, whisking constantly, until very thick, 3 to 4 minutes. Remove from the heat and stir in the butter and vanilla. Pour into a bowl. Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Chill until firm, about 45 minutes
FINISH THE CAKE
To finish the cake, place a cake layer on a cake plate or serving platter (reserving the most even layer for the top) and spread with cooled custard. Top with another layer of cake, then custard, then one more layer of cake. Cover the top and sides of the cake with the remaining custard. Coat the cake with the cake crumbs. Chill until ready to serve, at least 2 hours. Serve the same day.