For the cakes: Preheat the oven to 350 degrees F. Grease four 12- by 8- by 1-inch baking pans with cooking spray and line with parchment paper.
Prepare the dark chocolate cake according to package directions, using 1/4 cup canola oil and 3 eggs, and substituting the chocolate milk for the liquid. Bake in 2 of the prepared pans. Cool and remove the cakes from the pans.
Prepare the milk chocolate cake according to package directions, using the remaining 1/4 cup canola oil and 3 eggs, and substituting the cream soda for the liquid. Bake in the remaining 2 pans. Cool and remove the cakes from the pans.
While the cakes are baking, make the frostings.
For the milk chocolate frosting: Using an electric hand mixer, beat together the softened butter, softened cream cheese, confectioners' sugar, cocoa powder and chocolate milk.
For the dark chocolate frosting: Using an electric hand mixer, beat together the softened butter, confectioners' sugar, cocoa powder and chocolate fudge sundae topping.
For the chocolate glaze: Place the chocolate chips in a medium microwave-safe bowl and set aside. In a small saucepan, combine the cream and corn syrup. Cook over medium heat just until boiling. Pour over chocolate chips and stir until completely smooth. If not completely smooth, microwave on medium setting (50 percent power) for 2 minutes. Cool about 5 minutes or until the mixture thickens.
Cut each cake in half horizontally, to make 8 even layers. Place a dark cake layer onto a serving platter and frost the top only with some dark chocolate frosting. Place a milk chocolate cake layer on top and frost the top only with the milk chocolate frosting. Continue until you have 8 layers. Don't frost the top layer. Carefully pour the glaze over the cake, coating the top and letting it drip down the sides.