Heat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with parchment paper, leaving an overhang for easy lifting.
Mash the bananas in a large bowl with a fork until mostly smooth with a few small lumps.
Whisk in the melted butter, then the sugar, eggs and vanilla until well combined.
Sprinkle the flour, baking soda, salt and cinnamon over the top and fold gently with a spatula until just combined. Do not overmix.
Fold in the walnuts if using. Scrape the batter into the prepared pan and smooth the top.
Bake for 55 to 60 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. Tent loosely with foil for the last 15 minutes if the top browns too quickly.
Cool in the pan for 10 minutes, then lift out and cool on a wire rack for at least 30 minutes before slicing.