Why is my cupcake brownie like?
Baking powder contains both a chemical leavener (baking soda) and the acid required to activate it. It is normally used instead of baking soda in recipes where there isn't enough acid to activate baking soda. Cocoa powder is very acidic. What may have happened is that the combination of cocoa powder and baking powder contained too much acid for the baking soda, which produced too much leavening, inflating your batter so much that it then fell, resulting in the dense, brownie-like product you got.