Preheat your oven to 400 degrees Fahrenheit. Line a 12-cup muffin tin with paper liners or grease well.
In a medium bowl, whisk together the gluten-free flour blend, baking soda, baking powder, salt, and cinnamon. Set aside.
In a large bowl, whisk the melted butter and brown sugar until combined.
Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas and milk until just combined. Do not overmix.
Gently fold the dry ingredients into the wet ingredients until just incorporated. A few lumps are fine.
Fold in the chocolate chips, reserving a few for the tops if desired.
Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full. Sprinkle reserved chocolate chips on top.
Bake for 5 minutes at 400 degrees Fahrenheit. Then, reduce the oven temperature to 375 degrees Fahrenheit and continue baking for an additional 15 to 18 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
Notes
If your gluten-free flour blend does not contain xanthan gum, add 1/2 teaspoon to the dry ingredients.
For dairy-free muffins, use a non-dairy milk like almond or oat milk and dairy-free chocolate chips.
Use very ripe bananas for the best moisture and natural sweetness.