Cocoa powder is what remains after much of the cocoa butter is pressed from roasted cacao. Natural and Dutch-process powders differ chiefly in acidity, colour and flavour — a difference that can also affect a cake's leavening.
Natural cocoa
Natural cocoa has not been alkalised. It is relatively acidic, often lighter and more sharply fruity. Recipes that pair it with baking soda may be using that acidity as part of the chemical leavening system, not merely choosing a chocolate flavour.
Dutch-process cocoa
Dutch-process, alkalised or “processed with alkali” cocoa has been treated to reduce acidity. It is commonly darker and rounder in flavour. Very dark cocoa is not automatically stronger or better; colour depends on processing as well as cacao.
Can one replace the other?
In a sauce, frosting or recipe whose rise does not depend on cocoa's acidity, substitution is often mainly a flavour and colour decision. In a cake using baking soda, changing the cocoa can disturb the acid balance. A recipe built around baking powder may be more flexible, but the tested powder remains the reliable choice.
Cocoa is not drinking chocolate
Unsweetened cocoa powder is not the same as a sweetened hot-chocolate mix, which may contain sugar, milk powder and starch. Replacing one with the other changes far more than chocolate intensity.
