Follow the cake recipe on the back of the yellow cake mix box. Most yellow cake mix recipes call for the addition of eggs, water and oil. When the batter is mixed, add four tablespoons of cocoa powder. Mix in the cocoa till it is well incorporated in the batter.
If you are concerned that the cake will be crumbly because of the addition of cocoa, then add 1/4 cup of Greek yogurt. This will help add moisture to the cake and balance the addition of the extra dry ingredient. To intensify the cocoa flavor, add a cup of semi-sweet chocolate chips and chocolate pudding mix to your cake batter. The chocolate flavor in cakes is also intensified when brewed coffee is substituted for the water. Make sure to cool the coffee down prior to adding it to the mix, as hot coffee will scramble the eggs in the batter.
Bake the cake or cupcakes at 350 degrees Fahrenheit. Baking times will vary depending on the size of the cake pans. Insert a toothpick in the center of the cake to check if it's done.