Preheat oven to 350°F (175°C). Grease or line 12 standard-size muffin cups with paper liners (a silicone muffin pan also works).
In a large bowl, mash 3–4 very ripe bananas (about 1 1/2 cups) with a fork until mostly smooth.
Add 2 eggs, 1/2 cup softened butter, 1/2 cup brown sugar, 2 tablespoons milk, and 1 teaspoon vanilla extract to the mashed bananas. Whisk until the mixture is combined and fairly smooth.
In a separate bowl, whisk together 2 cups gluten-free flour, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/4 teaspoon salt.
Pour the dry ingredients into the wet ingredients and stir gently with a spatula until just combined—do not overmix.
Fold in 1/2–1 cup chocolate chips (or your chosen add-in) until evenly distributed.
Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Bake for 20–22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Let the muffins cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.