How to Make It
Preheat the oven to 375°. Butter two 9-inch round cake pans and coat lightly with flour. Line the bottoms with parchment paper. In a standing electric mixer fitted with the whisk, beat the 1 stick of butter with the shortening until creamy. Add the sugar and beat at medium speed until fluffy, about 3 minutes. Add the eggs one at a time, beating well between additions. Add the vanilla. At very low speed, beat in the cocoa powder, baking powder, baking soda and salt. Add the 2 1/4 cups of cake flour and the milk in 3 separate alternating batches, scraping down the side and bottom of the bowl occasionally.
Divide the cake batter between the prepared pans and smooth the tops. Bake in the center of the oven for about 30 minutes, until a toothpick inserted in the centers comes out with a few moist crumbs attached. Let the cakes cool for 15 minutes, then invert them onto a rack and let cool completely.
In a large saucepan, combine 2 1/2 cups of the water with the sugar, corn syrup and cocoa powder and bring to a boil, whisking constantly. In a bowl, whisk the cornstarch with the remaining 1/2 cup of water until smooth; whisk into the cocoa mixture. Cook over moderately high heat, whisking constantly, until very thick, 3 minutes. Off the heat, whisk in the butter, vanilla and salt. Scrape the filling into a bowl and press a sheet of plastic wrap directly onto the surface of the filling (you should have about 5 cups). Let cool, then refrigerate until firm, 45 minutes.
Using a serrated knife, halve each cake layer horizontally. Break up the less attractive top cake layer and transfer to a food processor; pulse into crumbs. Reserve the two cake bottoms and one smoother top.
Set one of the cake bottoms on a cake plate and spread with 1 1/2 cups of the filling. Top with the second bottom layer and another 1 1/2 cups of filling. Cover with the cake top and spread the remaining filling all over the top and side. Pat the crumbs all over the cake. Refrigerate for at least 1 hour before serving.
The chocolate cake can be refrigerated overnight.
Natural cocoa powder is one of two types of unsweetened cocoa. It’s bitter and adds intense chocolate flavor to the cake. Don’t use Dutch-process or other alkalized cocoa; when combined with baking soda, it can make a cake taste soapy.