Prep the pan and oven. Heat the oven to 350°F (175°C).
Line a 9x5 loaf pan with parchment, letting it hang over the long sides for easy lifting. Lightly grease the pan and parchment.
Mash the bananas. In a large bowl, mash the bananas until mostly smooth with small lumps. Measure to ensure you have about 1 1/4 to 1 1/2 cups.
More banana means extra moisture.
Whisk the wet ingredients. Add the eggs, oil (or melted butter), sugar, and vanilla to the bowl. Whisk until well combined and slightly frothy, about 30 seconds.
Combine the dry ingredients. In a separate bowl, whisk the gluten free all-purpose flour, almond flour, starch, baking soda, baking powder, salt, and cinnamon. This helps evenly distribute the leavening and prevent clumps.
Mix the batter gently. Add the dry ingredients to the wet and stir with a spatula just until no dry streaks remain.
If using chocolate chips or nuts, fold them in now. The batter will be thick but scoopable.
Rest the batter (short and sweet). Let the batter sit for 5–10 minutes. This quick rest hydrates the flour and improves texture in gluten free baking.
Fill and smooth. Scrape the batter into the pan and smooth the top. For a bakery-style split, run a buttered knife in a shallow line down the center.
Bake. Bake for 50–60 minutes, until the top is deeply golden and a toothpick inserted in the center comes out mostly clean, with a few moist crumbs.
If the top browns too fast, tent loosely with foil during the last 10–15 minutes.
Cool properly. Let the bread cool in the pan for 10–15 minutes, then lift it out to a wire rack. Cool completely before slicing for the best texture and clean slices.
Slice and enjoy. Use a serrated knife to cut even slices. Serve warm or at room temperature.