Using Cocoa in Place of Flour?

The reason Rose L-B suggests that you hydrate the chocolate cake with water as opposed to milk is that the latter contains lactic acid; added to chocolate, which is also acidic, you would need to adjust for excess acid. I refer to my above answer, which is predicated on using milk rather than water. Add some baking soda, use milk as your liquid, and you're golden.

If replacing a quantity of flour with cocoa powder, which is acidic, also add baking soda in a ratio of 1/2 teaspoon for every 8 ounces of acidic ingredient. A couple of tablespoons of cocoa powder weighs (rounding up) about 1/2 ounce. So add a generous pinch of baking soda to the dry ingredients before you sift them. You are going to sift, right?

When Rose Levy Beranbaum makes her Chocolate Butter Cake she uses a similar formula as for her Yellow Butter Cake. One of the main differences is the replacement of some of the cake flour with Dutch-processed cocoa powder measured by weight. The recipe for the yellow butter cake calls for 10.5 oz cake flour, whereas the chocolate version calls for 8.25 oz cake flour and 2.25 oz cocoa. The other main change is to use water in place of milk as the milk proteins affect the flavor of the chocolate. To get the most chocolate flavor she boils the water and mixes it with the cocoa, then lets it cool before proceeding. Hope that helps.

Here's an answer I thought was very instructive from The Art and Soul of Baking (http://www.knivescookslove.com/the_art_and_soul_of_bakin/2008/11/crazy-for-cocoa.html)
"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 a shortbread cookie you can use whatever cocoa you like."