Heat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, so the long sides hang over for easy lifting. Lightly grease the short sides.
Mash the bananas in a large bowl until mostly smooth. A few small lumps are fine.
Whisk in the eggs, oil (or butter), sugars, yogurt, and vanilla until the mixture looks cohesive and glossy.
In a separate bowl, whisk the gluten-free flour, baking soda, baking powder, salt, and cinnamon. This helps avoid pockets of leavening.
Add the dry ingredients to the wet. Stir gently with a spatula until just combined. If using nuts or chocolate chips, fold them in. The batter will be thick and a little sticky.
Pause for 10 minutes. This brief rest lets the gluten-free flour hydrate, which means a better crumb.
Scrape the batter into the pan and smooth the top. If you like, sprinkle a few extra nuts or chips over the surface.
Bake for 55–65 minutes, until the top is deep golden and a skewer inserted in the center comes out with a few moist crumbs. If the top is browning too fast, tent loosely with foil for the last 10–15 minutes.
Cool in the pan for 15 minutes, then lift out and cool completely on a rack before slicing. This is the hardest step, I know, but it sets the crumb.