The thing I will add to above is that Dutch processed cocoa powder is much darker and much less red. It also tends to be preferred by Europeans. It is the flavor you might associate with Oreo cookies (the chocolate cookie part I mean). I agree with the more complex part. They both have their uses.
Dutch process cocoa is treated with an alkali to neutralize the acids, resulting in a milder cocoa powder. Regular unsweetened cocoa is more bitter and more complex, and when mixed with baking soda in recipes will cause the batter/dough to rise.
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