Brown The Butter. Before you start the muffin batter, brown the butter. Dice the butter into cubes then add to a medium saucepan. Cook over medium heat 8-10 minutes, stirring constantly until the butter is golden brown and fragrant. Remove from the heat and pour into a heatproof bowl to cool to room temperature. (Get step-by-step instructions for browning butter here!) Combine Dry Ingredients. In a medium bowl, combine gluten-free flour, baking powder, baking soda, and salt. Whisk to combine, then set aside.
Combine Cooled Butter & Sugar. In a large bowl, combine cooled brown butter, brown sugar, and sugar.
Finish Wet Ingredients. Add the mashed bananas, sour cream, eggs, and vanilla. Whisk until evenly combined.
Add Dry Ingredients To Wet Ingredients. Add the flour mixture and the mini chocolate chips to the wet ingredients and stir until just combined.
Let Batter REST While You Preheat & Prep. If possible, let the batter rest 30-60 minutes at room temperature. (This will give the muffins a taller rise and a more tender texture!) While the batter rests, preheat the oven to 425 degrees F. and line a metal muffin pan with parchment paper liners.
Scoop & Garnish. After the batter has rested, use an ice cream scoop or extra large cookie scoop to divide the batter between 12 muffin cups. (You may have 1-2 muffins left for a second batch, depending on how long you let the batter rest!) Sprinkle the top of each muffin with coarse sugar or sparkling sugar and a few more chocolate chips, if desired.
Bake + Reduce Temp + Bake. Then, bake at 425 degrees F. for 5 minutes. Reduce the oven temperature to 350 degrees F (leave the muffins in the oven!) and bake an additional 12-15 minutes, or until the muffins are golden brown and a toothpick inserted in the center comes out clean with just a few moist crumbs.
Cool, Serve & Store. Let the muffins cool in the pan 3-5 minutes, then transfer to a wire rack to finish cooling completely. Enjoy right away and store leftovers in an airtight container or bag at room temperature 2-3 days or freeze in a freezer bag up to 2 months. (I recommend warming up leftover muffins for a few seconds in the microwave to revive their texture!)