Adding unsweetened cocoa powder to muffin mix?

I've never done much research with gluten-free flowers, but a rule I've always gone by with adding cocoa (or any other "flour"-type ingredient) to a recipe is by taking out 25% of the flour by weight and adding the cocoa.

You will have to reduce the amount of baking mix you use because cocoa acts like flour in absorbing liquids. If you just add cocoa without an associated adjustment in liquids, your muffins will be too dry and heavy. Check out this thread, https://food52.com/hotline/19632-using-cocoa-in-place-of-flour I'm guessing the mix is GF flours and leavening? Also check the Bob's Red Mill website for recipe ideas.