Preheat the oven to 350ºF. Line 12 cups of a standard muffin pan with paper liners.
In a large mixing bowl, combine the flours, sugar, baking powder, baking soda, xanthan gum, and salt. Set aside.
In a medium mixing bowl, whisk together the mashed bananas, sour cream, eggs, oil, and vanilla extract. Pour the wet ingredients into the dry ingredients. Stir to combine. Fold in the chocolate chips.
Divide the batter among the lined muffin cups. Bake for about 22 minutes or until golden brown and a toothpick inserted into the middle of a muffin comes out clean. Cool in the pan for 10 minutes, then transfer to a wire rack to finish cooling. Cool completely before serving. Store in an airtight container in the refrigerator.