In a large bowl, combine flours, baking powder, baking soda, xanthan gum and salt; set aside.

Peel bananas. In a separate bowl, mash bananas and brown sugar together until relatively smooth.

Add milk, vegetable oil, eggs, and vanilla, stir until well combined.

Add banana mixture and walnuts to the flour mixture, stir just until moistened (batter should be lumpy.)

Divide batter between 12 prepared muffin pan, filling each cup to almost full.

If you’d like, sprinkle some coarse sugar on each muffin top, and/or add some thinly sliced banana.

Bake in a 375 degree F oven for 18 to 20 minutes or until golden and a wooden toothpick inserted in centers comes out clean.

Cool in muffin cups on a wire rack for 5 minutes.

Remove from muffin cups; serve warm.