You're craving chocolate cake, but the only thing in your cupboard is a box of yellow cake mix. You don't have to give up. Your yellow cake mix can form the basis for a variety of other cake flavors, including chocolate. Cocoa adds chocolate flavor without changing the recipe enough that other ingredients need to be adjusted.
Open the box of cake mix and dump it into a mixing bowl. Measure 4 tbsp. of baking cocoa and mix it in with a fork. If you want a more intense chocolate flavor, use 5 tbsp. instead of four.
Add the eggs, water and oil called for in the directions on your cake mix box. Mix all of the ingredients together with a fork or with a mixer until the batter is smooth.
Transfer the batter to greased cake pans and bake according to the package directions.