Heat the oven to 350°F. Line a 9-by-5-inch loaf pan with parchment paper.
Mash the bananas in a large bowl. You should have about 1½ cups.
Whisk in the eggs, vanilla, and cooled melted butter until combined.
Add the gluten-free flour, sugar, baking soda, salt, and cinnamon. Stir until thoroughly combined. Fold in chocolate chips or nuts, if using.
Pour the batter into the prepared loaf pan. Sprinkle a few extra chocolate chips over the top if you like.
Bake on the center rack for 50–60 minutes, until the loaf has risen and the top is golden. If it’s browning too quickly, loosely cover it with foil. It’s done when a knife inserted in the center comes out clean, or the center reaches 165°F.
Turn off the oven and leave the loaf inside for another 5–10 minutes. Remove it from the oven and let it cool in the pan for about 30 minutes before slicing.