Pre-heat oven to 375°F and spray a muffin tin with non-stick spray.
In a large bowl, sift together flour, xanthan gum, baking powder and salt. Whisk in the brown sugar, breaking up lumps, and 1/2 cup of the walnuts.
In a separate bowl, whisk together eggs, milk, oil and vanilla. Stir the wet ingredients into the dry ingredients and mix until almost combined. Stir in bananas and mix until incorporated. Don’t over mix!
Transfer batter to the muffin tin, dividing evenly between the 12 cups. Bake at 375°F for 20 minutes or until tester comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack.