Preheat the oven to 375°F. Grease a 12-cup muffin pan.
In a large bowl, stir together the melted ½ cup unsalted butter, melted, 2 large eggs, and 1 ½ cups mashed ripe bananas. Stir in the 1 ½ cups gluten free flour, ½ cup brown sugar, 2 teaspoons baking powder, ¼ teaspoon baking soda, ½ teaspoon fine sea salt, and ½ teaspoon ground cinnamon until combined.
Fold in the chopped 1 apple, then divide the batter evenly between the muffin cups, filling each one about ⅔ full. Sprinkle with a little powdered sugar if desired.
Bake the muffins for 20 to 24 minutes, until the middle of a muffin springs back when lightly touched and a tester comes out mostly clean. (A few moist crumbs are OK.)
Remove the pan from the oven, let the muffins rest for 5 minutes in the pan, then turn them out onto a rack to cool completely. Muffins are great warm or at room temperature.
Notes
To peel or not to peel? You can leave the peel on the apple before chopping it up if you wish; it's entirely up to you! I like to peel my apple, but for a bit more texture, leave the peel on.
Rest the batter. Gluten-free flour always benefits from a rest before baking. At least 20 minutes is ideal, although letting it rest while the oven heats is also an option. This helps the flour to hydrate, making for a better baked muffin!
Fill the muffin cups ⅔ full for this recipe. While some muffin recipes advise filling the muffin cups to the brim, I've found that about ⅔ full is the sweet spot so that they rise nicely but don't overflow.
Refined sugar-free. Swap maple syrup or coconut sugar for the brown sugar.
Make it dairy-free. Substitute non-dairy butter for the unsalted butter.
Adjust the sweetness. Depending on how ripe your bananas are, you may wish to reduce the sugar by ¼ cup.
To Serve: Softened butter, cream cheese, or plain!