Heat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with a strip of parchment, leaving the ends overhanging.
In a large bowl, mash the bananas with a fork until mostly smooth, with a few small lumps.
Stir in the melted butter, brown sugar, egg, yogurt and vanilla until well combined.
Sprinkle the baking soda, salt and cinnamon over the wet ingredients and stir them in. Add the flour and fold gently just until no dry streaks remain. Fold in the nuts or chocolate chips, if using.
Scrape the batter into the pan and smooth the top. Bake for 55–65 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. If the top browns too quickly, cover loosely with foil after 40 minutes.
Cool in the pan for 15 minutes, then lift the loaf out and cool on a rack for at least 30 minutes before slicing.