If you're like me, when you read cocoa on a recipe you ignore the rest of the text, pick up your canister of cocoa and throw it in. But this can be a serious disaster! When it comes to cocoa not all are created equal. There are two main types of unsweetened coca and while substituting one for the other may not ruin your baked creation it can seriously detract from the flavor.
Dutch processed cocoa aka alkalized cocoa is treated with alkali to neutralize it's natural acids. It's reddish brown, has a mild flavor and dissolves well in liquid. It's neutral properties mean it will not react with baking soda and must be used with baking powder instead (unless you have sufficient acid in other ingredients to make up for it).
Natural unsweetened cocoa has a much richer, more bitter taste. It reacts with baking soda in a leavening reaction that causes your cake to rise. This stuff is intense and should be used accordingly: think brownies and deep, rich chocolate cake.
So next time you read cocoa on a recipe, check which one is called for. It pays to follow directions.