What's the difference? It's something many home bakers ponder and my ignore, but if you use the wrong one, you will end up with a significantly bitter product. Cocoa Powder is the remaining product after much of the cocoa butter is removed from the chocolate liquor. Common brands would be Hershey's or Nestles.
Dutch Processed Cocoa Powder has alkali added to neutralize the acids in natural cocoa powder.
Therefore, if you need Dutch Processed Cocoa Powder but only have natural cocoa powder, than you'll need to add "alkali" such as baking soda. Here's what you need to do according to www.joyofbaking.com
3 Tablespoon Natural Cocoa Powder + 1/8 teaspoon baking soda = Dutch Processed Cocoa Powder.
If you happen to have Dutch Processed Cocoa and need to have Natural Cocoa then you'll need to do the following:
3 Tablespoons Dutch Processed Cocoa Powder + 1/8 teaspoon EITHER Cream of Tarter, vinigar or lemon juice = Natural Cocoa Powder.
The photo to the right is our cocoa powder "loaded" SCYMIH cupcake found only here in the Poconos!