1/2cup chopped cashewsnuts are optional; choose your favorite if any, or use GF CF chocolate chips
Preheat oven to 350. Lightly grease a 9 x 5 inch loaf pan with coconut oil or butter and dust with rice flour. You can also line with parchment if you like instead. Both work well. Parchment lets you lift the loaf neatly out of the pan after baking.
If you have a stand mixer such as a KitchenAid, place the butter in it and cream til pale or white. While the mixer is running, in a separate bowl, blend the GF flours, baking soda, xanthan gum, and salt.
When the butter is pale or white, add the sugar. Beat til fluffy and smooth, about two more minutes. Add the eggs, one at a time, and blend thoroughly. Stir in the mashed bananas and mix well.
Add the dry ingredients and stir until just blended, then fold in nuts or chocolate chips. Spoon batter into loaf pan. Bake for 40 minutes and test for doneness, add more time if needed.