Cocoa powder comes from chocolate that has had most of the cocoa butter removed from it.

There are two types of cocoa powder.

The first kind is Dutch processed cocoa powder.
Dutch processed cocoa has been processed with alkali and is usually slighter darker than the natural cocoa powder.
It has a more delicate flavor.
It is also more readily absorbed into liquids, such as hot water or milk.
When baking with dutch processed cocoa you must use baking powder as your leavening.
Since this cocoa is neutral or slightly alkaline it will not react with baking soda.

The second type of cocoa powder available is natural. It can also be referred to as Non-Dutch processed cocoa.
It is more acidic so it can be used with baking soda as your leavening agent.

So, always make sure to follow the recipe.
If it says to use Natural cocoa, don’t use Dutch due to the type of leavening agent needed.

If you are lucky, you may be able to find a cocoa powder brand that combines both Dutch and natural cocoa so you always have the right kind on hand.