Preheat and prep. Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment, leaving overhang on the long sides for easy lifting.

Mash the bananas. Mash the 3 bananas in a large bowl with a fork until mostly smooth with a few small lumps left. You should have about 1½ cups of mashed banana — this is what keeps the loaf moist, so don't skimp.

Mix the wet ingredients. Stir the melted butter into the mashed banana. Add the sugar, egg, and vanilla, and whisk until combined. The mixture will look glossy and slightly foamy on top after about 30 seconds of whisking.

Add the dry ingredients. Sprinkle the baking soda, salt, and cinnamon (if using) over the wet mixture, then add the flour. Fold with a spatula just until no dry streaks remain, about 15 to 20 strokes. Stop as soon as the flour disappears — overmixing develops gluten and makes the loaf tough and rubbery instead of tender.

Fold in add-ins. If using nuts or chocolate chips, fold them in now with 2 to 3 more strokes.

Pour and bake. Pour the batter into the prepared pan and smooth the top. Bake for 55 to 65 minutes. Check doneness at 55 minutes: a toothpick inserted in the center should come out with a few moist crumbs, not wet batter, and the internal temperature should read 200 to 205°F on an instant-read thermometer. The top will be deep golden brown and may have a single long crack down the center — that's normal, not a flaw.

Cool before slicing. Let the loaf cool in the pan for 10 minutes, then lift it out onto a wire rack using the parchment overhang. Cool for at least 30 minutes before slicing — cutting it warm makes the crumb gummy, even though it smells impossible to resist.