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.
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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.