Cake base: Prepare and bake the cake according to the package directions. Grease & flour the 9x13 baking pan, paying special attention to the bottom so that the cake comes out easily. Let it cool for 10 minutes before inverting onto a wire rack to cool completely.
Buttercream filling: In a mixing bowl using an electric mixer, cream the butter, shortening, flour and sugar until it's light and fluffy.
Heat the milk for 1 minute in the microwave, test the temp and reheat if necessary, until it reaches 140°F. Add the milk to the creamed mixture, beating until the sugar has dissolved. Stir in the vanilla extract.
Cut the cake in half, so you have two horizontal layers; then spread the filling over the bottom cake layer. Top it with the remaining cake layer.
Chocolate glaze: In a saucepan, combine the sugar, cocoa, cornstarch, and water; whisk until smooth. Bring the mixture to a boil over medium heat, whisking constantly. Cook and stir for 1-2 minutes or until thickened. Remove from the heat; add in butter and vanilla extract, whisk until glaze is smooth. Cool to lukewarm.
Spread the glaze over the top of the cake and let it stand until set. When ready to serve, slice and enjoy!