In the mid-nineteenth century, a Dutch chocolate manufacturer came up with a process by which he could better control and standardize the color and flavor of cocoa. The process, which involves washing the cocoa (before or after grinding) in an alkaline solution, became known as Dutch-processing.
The resulting cocoa is consistently darker in color, mellower in flavor, and less acidic than the natural (non-alkalized) powder. In cakes and brownies, the Dutch-processed cocoas tend to produce moister and deeper colored baked goods—an advantage that makes it a favorite of many pastry chefs.
2-1/4 oz. = 3/4 cup
You can substitute natural cocoa powder for Dutch 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.
Most supermarkets carry Dutched cocoa powder, and it will say it's Dutched right on the box. There are flavor variations among brands. You might find that you love the complex flavor of premium brands, such as Merckens and Valrhona, or you might prefer the familiar flavor of the supermarket brands like Hershey's and Nestlé.
Stored in a cool, dry place, cocoa powder will keep almost indefinitely.