Preheat the oven to 375 degrees.Butter and flour 2 (9-inch) cake pans.
Cut 2 circles of parchment paper or waxed paper to fit the bottoms of the pans, then press them in.In a mixer, cream the butter and shortening together.
Add the sugar and mix until light and fluffy.
Add the eggs one by one, mixing after each addition.
Add the vanilla, cocoa, baking powder, baking soda, and salt and mix.Then add, alternating, about 1/3 of the cake flour, then about 1/3 of the milk until finished.
Pour into the prepared pans and bake for 30 to 35 minutes or until a tester inserted into the center comes out clean,
Pour 2 1/2 cups of the water, the sugar, corn syrup and cocoa powder into a large non-reactive saucepan and bring to a boil over medium-high heat, whisking occasionally.Meanwhile, in a small bowl, whisk the remaining 1/2 cup of water and the cornstarch.
Whisk into the cocoa mixture in the saucepan and return the mixture to a boil, 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.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 the final 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.
