About Dutch Processed Cocoa
Cocoa has natural acidity, but Dutch-processed cocoa has been treated to neutralize that acidity (with a natural alkali), which also reduces bitterness in the cocoa.

In baking, it is best to use the type of cocoa called for in the recipe (if one is specified). When cocoa that is not Dutch-processed (so, an acid) is used in recipes, generally the recipe will call for baking soda (an alkali), then a leavening action results, causing the batter to rise when baked. But if the cocoa IS Dutch-processed, then its acid qualities have been neutralized and the batter will not rise in reaction to the baking soda (unless there are sufficient quantities of other acid ingredients, like buttermilk or molasses). 


So, when Dutch-processed cocoa is called for, the recipe often includes baking powder, rather than baking soda. This is because baking powder is a combination of an alkali (baking soda) and an acid (often cream of tartar), which together assure the reaction that causes leavening, regardless of other ingredients. This is part of the brilliance of the invention of baking powder.