Can I add cocoa to chocolate to approximate a higher % of cacao? I have a recipe that I'd like to make that calls for 3oz of 72% cacao chocolate. I have plenty of 60% cacao chocolate, and I have Hershey's cocoa powder. The recipe is just for brownies, so absolute precision and supreme quality isn't required, but I would like to get the right level of "chocolatyness". 3oz = 85g. Going from 72% cocao to 60% cacao would leave me with a deficit of just over 10g of cacao. If cocoa is basically 100% cacao (is it?), then it would seem that just adding 10g of cocoa would be close enough to "right".

Since I'm dealing with chocolate, not anthrax, I'm disinclined to worry about the 10g (or whatever) surplus total chocolate weight. Would the semi-substitution work?
Sure, you can add cocoa - but think of it as adding it to the brownies, not to the chocolate. And the thing to worry about isn't extra weight, but rather whether you're adding enough dry cocoa to upset the dry/water balance in the recipe. With only 10g and something flexible like brownies, it should be fine, but you'd have to be more careful with larger amounts or more sensitive baked goods like cookies.