The issue isn't sweetened vs. unsweetened; it's Dutch process vs. notThe questions above suggest there's some confusion about exactly what "Dutch process" and not-Dutch process cocoa powder mean. Here's some information from a website called "whatscookinginamerica.net:"
I'm confused! Can you tell me the difference between Dutch-Process Cocoa and Unsweetened Cocoa? Are they interchangeable in a recipe? - Nancy (1/10/04)
Both types of cocoa powder are unsweetened and therefore bitter when tasted alone.
Dutch-Process Cocoa or Alkalized Unsweetened Cocoa Powder has been treated with an alkali to neutralize its natural acidity. Because it is neutral and does not react with baking soda, it must be used in recipes calling for baking powder, unless there are other acidic ingredients in sufficient quantities used. It has a reddish-brown color, mild flavor, and is easy to dissolve in liquids.
Unsweetened Cocoa has a complex chocolate flavor while the Dutch-process is darker and more mellow. Its intense flavor makes it well suited for use in brownies, cookies and some chocolate cakes. When natural cocoa (an acid) is used in recipes calling for baking soda (an alkali), it creates a leavening action that causes the batter to rise when placed in the oven.
It is important to use the type of cocoa specified in a recipe because it may affect the recipe's balance of acid. If you must substitute, use the following formula:
Substituting Unsweetened Cocoa for Dutch-Process Cocoa - 3 tablespoons (18 grams) Unsweetened Cocoa Powder plus a pinch (1/8 teaspoon) baking soda.
Substituting Dutch-Process Cocoa for Unsweetened Cocoa - Leave out any baking soda called for in the recipe.
Hope this helps.