Preheat oven to 175°C (350°F). Grease a 23 × 13cm (9 × 5 inch) loaf pan and line with parchment, leaving overhang on both long sides.
Mash bananas. Peel all five into a large bowl. Mash thoroughly with a fork until mostly smooth — small chunks are fine. Drain any pooled liquid if bananas were frozen-thawed.
Add wet ingredients. Add the cooled melted butter, brown sugar, eggs, and vanilla. Stir until no streaks of egg remain — about 20 strokes.
Add dry ingredients. Sprinkle flour, baking soda, salt, and cinnamon over the wet mixture. Fold with a spatula using 12 to 15 strokes. Stop the moment no flour streaks remain. Do not overmix.
Pour and score. Pour batter into the prepared pan. Smooth the top. Optional: score a line down the center with a sharp knife.
Bake 60 to 70 minutes. At 45 minutes, check — cover loosely with foil if browning too fast. Test at 60 minutes with a toothpick; done when it comes out clean or with moist crumbs only.
Cool completely. Cool in the pan 10 minutes. Lift out using the parchment and transfer to a wire rack. Cool at least 30 minutes before slicing.