Preheat oven to 350 degrees F 1. Make the cake. Butter two 9-inch cake pans; dust them with flour and tap out the excess. Set aside. 2. Stir the cocoa with some of the milk to form a paste. Stir in the rest of the milk, and beat with a whisk until the mixture is smooth. Set aside. 3. In the bowl of an electric mixer combine the butter, shortening, sugar, eggs, vanilla; beat for 1 minute, or until the mixture is fluffy. 4. Sift together the flour, baking powder, baking soda, and salt. Add the mixture, alternately with the cocoa mixture, to the contents of the bowl, beating between additionsl Begin and end with the flour mixture, and beat only until the dry ingredients are absorbed. Divide the batter between the cake pans and smooth the tops with a spatula to even them. 5. Bake for 35 to 40 minutes, or until the layers shrink from the sides of the pans and the tops spring back when gently pressed with a fingertip. The cake is dense and moist, so be careful not to overbake it. Cool the layers on wire racks for 10 minutes, then carefully invert them onto the racks. Turn right side up and let cool completely. 6. Make the pudding. Combine the sugar, cornstarch, and salt in a small heavy saucepan. Gradually add the milk, mixing thoroughly with a wire whisk. Add the chocolate. Place over moderate heat and cook, stirring constantly, until the mixture thickens and bubbles for 3 minutes. Remove from the heat and stir in the vanilla. Pour into a small bowl and put plastic wrap or wax paper directly on the surface to prevent a skin from forming. 7. Make the frosting. Melt the chocolate in the top of double boiler. Remove it from the heat and cool slightly. In a medium bowl, beat the butter, vanilla, and eggs until well mixed. The mixture will not blend completely-do not worry. Gradually beat in the sugar, about 2 tbl at a time, beating well after each addition. Beat in the melted chocolate. Chill the frosting while assembling the rest of the cake, about 15 minutes. 8. Cut each cake layer in half horizontally, using a serrated knife. You now have 4 layers, 3 for the cake assembly and 1 for the outside crumbs. Place one of the layers in the food processor bowl and pulse-chop to make crumbs; set aside.(Or break up with your hands.) 9. Sandwich the remaining 3 layers with the chocolate pudding filling, assembling the cake on a cardboard circle or the bottom of a springform pan, if possible. Frost the side and top of the cake woth the chocolate frosting. Working over a baking sheet lined with aluminum foil or wax paper, hold the cake in the palm of your hand. With the other hand, gently press the cake crumbs all over the top and sides of the cake, pressing them to adhere. Pick up any crumbs that drop and press them back on.