Preheat the oven to 190°c and spray a standard 12 cup muffin tray with non-stick cooking spray or line with paper liners.
In a large bowl, whisk together the flour, xanthan gum, bicarbonate of soda, baking powder, and salt. Whisk in the brown sugar, breaking up lumps.
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.
Transfer the batter to the muffin tray, dividing evenly between the 12 cups. Bake for 20 minutes or until a toothpick inserted into the centre of a muffin comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Store muffins in an airtight container up to 3 days.