Pre-heat oven to 350 degrees and spray a 9x5in loaf pan with non-stick spray.
In the bowl of a stand mixer fitted with a paddle attachment, or hand mixer, cream together butter, granulated sugar and brown sugar.
In a separate bowl, sift together flour, xanthan gum, baking soda, baking powder and salt. Stir to combine.
Add egg and vanilla to the butter/sugar and beat until combined.
Beat in mashed banana.
Add half of the flour mixture to mixture and mix until just combined.
Mix in 1/3 C of the milk until combined.
Add the remaining flour mixture, mixing until combined, followed by the remaining milk and mix until all ingredients are combined.
Transfer batter to the prepared loaf pan and bake at 350 degrees for 55 min to 1 hour or until tester comes out clean.
Notes
Oven temps often vary so check for doneness after 55 min. In my oven, this banana bread takes 1 hour to bake. You can substitute regular all-purpose flour for the gluten-free all-purpose flour and omit the xanthan gum to make this a regular banana bread. Please be sure to use gluten-free ingredients when following a gluten-free diet.