Cocoa is the dry powder that remains after part of the cocoa butter is removed from chocolate liquor. Dutch process cocoa, or dutched cocoa, is processed with an alkali. It is slightly darker, smoother in flavor, and more easily dissolved in liquids than is natural cocoa.
Natural cocoa is somewhat acidic. When it is used in such products as cakes, it is possible to use baking soda (which reacts with acid) as part of the leavening power.
Natural cocoa
Dutched cocoa, on the other hand, is generally neutral or even slightly alkaline. Therefore, it does not react with baking soda (see table). Instead, baking powder is used as the sole leavening agent. If you are substituting dutched for natural cocoa, you must increase the baking powder by 1 ounce (30 g) for each ½ ounce (15 g) soda omitted.