I would never have thought of this one:
1. Black beans for flour
Swapping out flour for a can of black beans (drained and rinsed, of course) in brownies is a great way to cut out the gluten and fit in an extra dose of protein, Plus, they taste great. When baking, swap out 1 cup flour for 1 cup black bean puree (about a 15oz can).
Do It And How 