Start by preheating your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan to prevent the cake from sticking.
In a large mixing bowl, follow the instructions on the cake mix box, but make one crucial substitution — replace the water with 1 cup of Coca Cola. This gives the cake that signature soda flavor and enhances the overall richness.
Pour the cake batter into the prepared pan and bake according to the box instructions, usually around 30-35 minutes, or until a toothpick inserted into the center comes out clean. While your cake is baking, let’s move on to preparing the Coca Cola frosting.
In a medium-sized saucepan, combine the butter, unsweetened cocoa powder, and Coca Cola. Heat over medium heat, whisking constantly, until the butter melts and the mixture comes to a gentle boil.
Once it starts boiling, remove the saucepan from the heat and immediately stir in the sifted confectioners sugar. Whisk until the mixture is smooth and free of lumps.
Stir in the vanilla extract for that extra layer of flavor. If you’re using pecans, fold them in now, making sure they’re evenly distributed in the frosting. This gives the cake a delightful crunch.
When the cake is done baking, remove it from the oven and immediately pour the hot Coca Cola frosting over the warm cake. The frosting will spread easily and begin to firm up as it cools.
Allow the cake to cool completely in the pan. As it cools, the frosting will harden into a glossy, sweet shell that pairs perfectly with the soft, moist cake.