
Casein as a powder is much more water-absorbent and baking-oriented and is therefore allowing that dense brownie texture to happen, changing to whey would most likely make it so that the brownies would dry out, puff up, and become closer to muffins than brownies. The closest I would try to do would be like a 50/50 casein/whey blend where the textures would equal out to something manageable but that’s unknown territory for me so take my words with a grain of salt.