Preheat oven to 325ºPlace banana in a blender or food processor and pureé.Add brown sugar, yogurt, butter, eggs and vanilla, and blend until all is well incorporated.In a mixing bowl, add flour mix, xanthan gum, baking powder, baking soda, and salt, and mix.Pour banana mixture into mixing bowl with flour mixture, being sure to scrape all the contents out with a rubber spatula.Mix ingredients together until incorporated.Fold in chocolate chips.Pour batter into pastry bag, or zip-top bag (cut corner open to let batter out.)Pour batter into doughnut iron, close lid and bake for five minutes.OR:Grease doughnut pan.Squeeze batter to fill each mold to about ¾ full.Bake 12-15 minutes.Remove from pan.Mix cinnamon and white sugar together.Dip the top of each doughnut into melted butter, and then cinnamon-sugar mixture.Enjoy!