1. Preheat oven to 350°F. Line a 9-by-5-inch loaf pan with aluminum foil and coat foil with cooking spray.
2. Beat butter and sugar with an electric mixer on high speed until mixture is light yellow and fluffy. Add eggs one at a time, mixing on medium until thoroughly combined.
3. Add banana and 1⁄3 cup water and mix on low for 1 minute. Sprinkle brown rice flour, millet flour, baking soda, salt, xanthan gum, and baking powder over batter. Add nuts, if desired. Stir batter just until ingredients are combined.
4. Spoon into pan and bake for 50 to 55 minutes or until center of loaf springs back to the touch.
5. Transfer pan to a wire rack and allow to cool for 5 minutes. Lift bread out of pan and let it cool completely on rack.