(Gluten-Free Flour Mix: 2 1/2 C. Rice Flour, 2/3 C. Potato Starch, 1/3 C. Tapioca Starch; Mix well. Use 2 cups of mix for recipe) Pre-heat oven to 400 degrees F. Grease and dust a loaf pan with GF Flour Mix. In a bowl, mash bananas thoroughly. Dump in dry ingredients, in no particular order. Add vanilla, oil, softened butter, and enough milk to make a very thick batter (thinner is ok, it will just take a little longer to bake). Pour into prepared loaf pan and spread evenly. Sprinkle demerara sugar evenly over top (optional). Bake about 40 minutes, or until cake tester comes out clean.