Preheat the oven to 350 degrees F. Lightly grease a 9”x13” cake pan and set aside.
In a small pot add the butter. Then add 9 oz of coca-cola and ½ cup cocoa powder and whisk together. Bring to a low boil over medium heat whisking frequently. Once it begins to bubble, remove from the heat and set aside.
In a large bowl mix the flour, sugar and baking soda together to combine with a flat whisk or spatula.
Slowly pour the cocoa mixture into the dry ingredients and mix until combined.
Add the eggs, buttermilk and vanilla extract, mix again until no dry items remain.
Pour into the prepared 9”x13” pan and bake at 350 degrees for 20-25 minutes or until a toothpick inserted in the center comes out clean. Set aside in a ventilated area to cool to room temperature.
In a large sauce pan over medium heat add ½ cup (1 stick) butter and melt. Add ½ cup cocoa and the remaining 3 oz of coca-cola to a low boil.
Once it boils, remove from the heat and carefully whisk in the powdered sugar 1 cup at a time until smooth. Let the frosting sit for about 5-10 minutes to thicken slightly.
Pour the frosting over the cake and spread. Let the frosted cake set for 1 hour. Refrigeration is optional, room temperature is best. Slice and enjoy with chocolate drizzle or vanilla ice cream.
Notes
For the best flavor and texture, regular Coca-Cola is recommended as written. Diet or zero-sugar sodas contain different sweeteners and may alter the flavor and consistency of both the cake and frosting.
Once the frosting has fully set, cover the cake tightly and store it at room temperature for up to 2 days.