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1
make the cake Preheat the oven to 375.
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2
Butter two 9-inch round cake pans and coat lightly with flour.
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3
Line the bottoms with parchment paper.
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4
In a standing electric mixer fitted with the whisk, beat the 1 stick of butter with the shortening until creamy.
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5
Add the sugar and beat at medium speed until fluffy, about 3 minutes.
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6
Add the eggs one at a time, beating well between additions.
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7
Add the vanilla.
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8
At very low speed, beat in the cocoa powder, baking powder, baking soda and salt.
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9
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.
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10
make the cake Divide the cake batter between the prepared pans and smooth the tops.
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11
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.
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12
Let the cakes cool for 15 minutes, then invert them onto a rack and let cool completely.
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13
meanwhile, make the filling 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.
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14
In a bowl, whisk the cornstarch with the remaining 1/2 cup of water until smooth; whisk into the cocoa mixture.
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15
Cook over moderately high heat, whisking constantly, until very thick, 3 minutes.
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16
Off the heat, whisk in the butter, vanilla and salt.
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17
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).
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18
Let cool, then refrigerate until firm, 45 minutes.
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19
meanwhile, make the filling Using a serrated knife, halve each cake layer horizontally.
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20
Break up the less attractive top cake layer and transfer to a food processor; pulse into crumbs.
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21
Reserve the two cake bottoms and one smoother top.
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22
meanwhile, make the filling Set one of the cake bottoms on a cake plate and spread with 1 1/2 cups of the filling.
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23
Top with the second bottom layer and another 1 1/2 cups of filling.
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24
Cover with the cake top and spread the remaining filling all over the top and side.
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25
Pat the crumbs all over the cake.
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26
Refrigerate for at least 1 hour before serving.
