1 Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9 inch pans. Sift together the flour, baking powder, baking soda and 1/2 teaspoon salt. Set aside. In a separate small bowl, gradually stir 1 cup milk into 1/3 cup of cocoa, then whisk until the mixture is smooth.
2 In a large bowl, cream together 1/2 cup butter, 1/4 cup shortening and 2 cups sugar until light and fluffy. Add 3 eggs one at a time, beating well with each addition, then stir in 1 teaspoon vanilla. Add the flour mixture alternately with the milk mixture; beat well. Spread the batter evenly into the prepared pans.
3 Bake in the preheated oven 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 over-bake it. Cool the layers in the pans for 10 minutes, then carefully invert them onto the racks and cool completely.
4 To make the filling: In a small bowl, whisk together 2/3 cup sugar, 2 tablespoons cornstarch and 1/4 teaspoon salt. In a saucepan over medium heat, combine 1 1/2 cups milk with sugar mixture. Add 3 squares chopped chocolate, and bring the mixture to a boil, stirring constantly. Boil until the mixture is thick and bubbly, about 3 minutes. Remove from the heat, and stir in 1 teaspoon vanilla. Pour into a bowl and cover with plastic wrap directly on the surface, to keep the pudding from forming a skin. Refrigerate until cool.
5 To make the frosting: In a medium bowl, beat 1/2 cup butter, 1 1/2 teaspoons vanilla and 3 eggs until light and fluffy. Add the confectioners’ sugar two tablespoons at a time, beating well after each addition. Beat in the melted chocolate until well blended. Refrigerate for 15 minutes while you fill the cake.
6 To assemble the cake: Using a long serrated knife, slice each layer in half horizontally, making 4 layers. Spread the filling between 3 of the layers of cake. Frost cake with the frosting, then crumble the remaining cake layer. Stick crumbs to the entire frosted surface of the cake. Sprinkle any leftover crumbs over the top of the cake. Store in a cool place, and serve within 24 hours.