Alkalized (Dutch-processed) cocoa powder has been treated with an alkali solution to reduce the cocoa’s acidity, which gives it a darker color and mellower flavor. (The alkalizing procedure was developed in the Netherlands by chemist Coenraad Van Houten, hence the term “Dutch-processed.”) European bakers use it because it’s the type that’s most readily available in their markets; and it also gives their baked goods a richer, darker color. The most common brand is Droste. Hershey’s also makes a “European-style” (i.e., “Dutch-processed”) cocoa, which is sold in a silver box. If you look closely at the labels of ingredients of other brands, you may be surprised to find the words “alkalized” or “Dutch-processed” in the fine print.
The reduced acidity of Dutch-processed cocoa powder does not affect most European recipes for baked goods, because they don’t usually include chemical leaveners like baking powder or baking soda. But if you’re making a traditional American recipe with baking soda, don’t substitute Dutch-processed cocoa for regular, or you might not have enough acid in the recipe to make the cake rise.