Preheat oven to 350. Spray the bottom of a 9 x 9 baking dish with cooking spray.
With an electric mixer combine the first five ingredients until smooth. Add in the flours, sweetener, and chopped cocoa butter. Spread in the prepared baking dish. Bake for 25 minutes until the center no longer jiggles and the edges are golden.
Cool completely and then chill in the refrigerator for at least 2-3 hours before cutting.