Cocoa powder is made when chocolate liquor is pressed to remove 3/4 of its cocoa butter. The remaining cocoa solids are processed to make fine unsweetened cocoa powder. There are two types of unsweetened cocoa powder: Natural and Dutch-processed.
Actually, there's nothing very Dutch about Dutch Processed Cocoa. It's only called a Dutching process because the person who invented it, Coenraad J. van Houten, was a 19th century Dutchman who pioneered the use of the hydraulic press to defat chocolate liquor. Van Houten's solution lay in simple chemistry. Cocoa in its natural state is slightly acidic, as indicated by its pH value of around 5.4. By soaking the cocoa nibs in a basic (or alkaline) solution, he found he could raise the pH to 7 (neutral) or even higher. The higher the pH, the darker the color. And, the acids present in natural cocoa were neutralized, reducing its harshness.
Planning to bake with cocoa? Here's advice from David Lebowitz, the King of Chocolate.
Because natural cocoa powder hasn’t had its acidity tempered, it’s generally paired with baking soda (which is alkali) in recipes. Dutch-process cocoa is frequently used in recipes with baking powder, as it doesn’t react to baking soda like natural cocoa does.
Many classic American recipes, like Devil’s Food Cake, use natural cocoa powder. There is also a reaction between natural cocoa powder and baking soda that occurs in recipes, which creates a reddish crumb, like Devil’s Food Cake.
There are exceptions to each, of course. And according to Fine Cooking magazine, “You can substitute natural cocoa powder for Dutch-process in most recipes (though not vice versa). Flavor and texture can be affected, but generally only in recipes calling for 3/4 cup or more.” However when a batter-based recipe calls for natural cocoa powder, do not use Dutch-process cocoa powder. But I always advise folks to follow what the recipe says. For sauces and ice creams, they can be swapped out. For cakes and cookies, I don’t recommend it, as your results may not be the same if you make substitutions.
If a recipe calls for either, the main different is that Dutch-process cocoa will give a darker color and a more complex flavor whereas natural cocoa powder tends to be fruitier tasting and lighter in color.
Here are a few cocoas I like that are great in brownies, devil's food cake, and other chocolate baked goods: King Arthur Flour Double-Dutch Dark Cocoa, Callebaut, Guittard, Valrhona, Ghirardelli, and Trader Joe's.
When used alone in cakes, cocoa powder gives a full rich chocolate flavor and dark color. Cocoa powder can also be used in recipes with other chocolate (unsweetened or dark) and this combination produces a cake with a more intense chocolate flavor than if the cocoa wasn't present.
Most recipes call for sifting the cocoa powder with the flour but to bring out its full flavor, combine the cocoa powder with a small amount of boiling water. (If you want to try this in a recipe, substitute some of the liquid in the recipe for the boiling water.)
As I mentioned above there are two types of unsweetened cocoa powder: Natural and Dutch-processed. When in doubt, use the type specified in the recipe. Some prefer using Dutch-processed cocoa as a slight bitterness may be tasted in cakes using natural cocoa and baking soda.
Another Tip: Don't confuse unsweetened natural and Dutch-processed cocoa powder with sweetened cocoa drink mixes. They are not the same thing.
O.K. all the above is basic baking cocoa information. For me, though, the reality is that natural and Dutch processed cocoa powder are pretty much interchangeable. There are very few recipes that are thrown off by the presence or absence of the acidity of cocoa powder. In fact, many of the ingredients you regularly use in baking are slightly acidic, so even recipes that seem to rely on the acidity of cocoa powder to produce leavening are getting their acidity from milk, butter, egg yolks, honey (sugar is neutral), etc, and the recipe should turn out just fine whichever cocoa you use-- Dutch process or natural cocoa powder (but be sure and read David Lebowitz's comments above).