Preheat your oven to 400 degrees F. 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 this dry mixture 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.
Add the mashed bananas and milk to the wet ingredients. Mix until just combined. Do not overmix.
Gently fold the dry ingredients into the wet ingredients using a spatula until just combined. A few streaks of flour are acceptable.
Fold in 3/4 cup of the chocolate chips.
Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. Sprinkle the remaining 1/4 cup of chocolate chips over the tops.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Use a high-quality gluten-free flour blend that contains xanthan gum for the best structure.
Ensure your bananas are very ripe (lots of brown spots) for maximum sweetness and moisture.
To prevent dry muffins, stop mixing the batter as soon as the flour disappears.
These muffins freeze well for make-ahead snacks. Cool completely before storing in an airtight container.