Bake the cake in a 9 x 13-inch pan according to package directions, BUT replace the water with an equal amount of Coca Cola.
While the cake is still warm, prepare the Coca Cola Frosting.
Make the Coca Cola Frosting
In a large saucepan, bring butter, cocoa powder, and Coca Cola to a boil, whisking regularly. Remove from heat and mix in powdered sugar and vanilla until smooth. Fold in the pecans.
Ice the Cake
Pour the hot icing over the warm cake. The icing will firm up and harden into a sweet shell over the cake as it cools.
Notes
Check your package of chocolate cake mix to be sure that you're using the correct ingredients, since every brand differs slightly.
Pour the Coca Cola frosting over a warm cake.
Allow plenty of time for the cake to cool completely in the baking pan and for the icing to set before slicing and serving.
I have not tested this recipe with Diet Coke, as the flavor will differ. If you give it a shot, please let us know!
Swap out the Coke and use root beer instead.
Serve each slice with a scoop of vanilla ice cream, a drizzle of chocolate sauce, and maraschino cherries for real treat!