Cocoa powder comes in two main varieties, natural and Dutch process, and they are not always interchangeable. Acid is an important part of the reactions in baking and the two types of cocoa powder vary greatly in their acidity.
Natural cocoa powder is made from cocoa beans that are roasted and then ground into a fine powder after most of the fat is removed. Natural cocoa powder is acidic and often paired with baking soda (an alkali) in recipes to balance the flavor and create carbon dioxide bubbles that help baked goods rise in the oven.
Dutch process cocoa powder is made in a similar process except using cocoa beans that have been washed in a potassium solution to neutralize their acidity. Dutch process cocoa powder is darker in color, less acidic and has a more mellow, smoother flavor than natural cocoa powder. Since Dutch process cocoa powder is not acidic, it will not react with baking soda to create lift and rise in baked goods. It also will not balance the pH of the baking soda and can result in a bitter flavor.