I want a cake-like cookie, almost like a whoopie pie, but I need vegan and gluten-free. I played around with a banana cake recipe today involving multi-purpose flour, tapioca starch, baking powder, bananas, light brown sugar, vegan butter, and flax seed/water mixture (in place of the egg). The flavor was great but the texture was not what I wanted. The insides were cake-y (though a little packy), but the outsides turned out tough, especially on the bottoms. What would have caused tough cookies?