Cyn, I found this. It's actually the reverse but it should help you.
When modifying a recipe to add cocoa powder, start by removing a couple of tablespoons of flour and replacing it with cocoa powder. This may not seem like much, but cocoa is intense chocolate flavor. You can always add another tablespoon or two if you like, but keep in mind that you want the chocolate flavor to balance the sugar in the recipe, and the more cocoa you add, the more sugar and moisture you’ll need as well. Cocoa powder is like a sponge, sucking up moisture, and if you don’t adjust for its presence, you’ll end up with a dry pastry. If adding more than a couple of tablespoons, you’ll want to add a couple additional tablespoons of liquid, as well. You’ll also need to keep in mind the type of cocoa powder you are using, and match it to the chemical leavener (if any) in your recipe. If you are adding cocoa powder to a cake, muffin or scone batter, be sure to follow the guidelines above for matching the type of cocoa to the leavener you have in your recipe. If there is no chemical leavener in the recipe, such as the Chocolate Earl Grey Shortbread Coins below, you can use whatever type of cocoa you like. This applies to hot cocoa, chocolate pudding, and chocolate tart dough as well. For a double dose of pure chocolate flavor, try the Chocolate Cocoa Nib Shortbread Cookies variation in the recipe below.