Preheat oven to 350°F and spray a 9x5 inch loaf pan with non-stick spray or 5 mini loaf pans.

In a large bowl, whisk together flour, xanthan gum, baking soda, baking powder and salt. Whisk in brown sugar and break up any clumps. Mix in peanut butter chips and set aside.

In a medium bowl (or large measuring cup), whisk together milk, oil, egg and vanilla extract. Add the wet ingredients and mashed banana to the dry ingredients and mix until just combined.

Transfer batter to the prepared baking pan or mini pans and bake at 350°F for 55 minutes to 1 hour for a regular sized loaf, or 35 minutes for mini loaves, or until tester comes out clean.