Prep the pan and oven: Heat oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with a parchment sling for easy removal.
Mash the bananas: In a large bowl, mash bananas until mostly smooth with a few small lumps. You should have about 1 1/3 to 1 1/2 cups.
Whisk the wet ingredients: Add melted butter, brown sugar, eggs, yogurt or sour cream, and vanilla.
Whisk until well combined.
Combine the dry ingredients: In a separate bowl, whisk flour, baking soda, salt, and cinnamon until evenly mixed.
Bring it together: Add the dry ingredients to the wet. Stir gently with a spatula until just combined. If using nuts or chocolate chips, fold them in now.
Do not overmix.
Pan and top: Pour batter into the prepared pan. Smooth the top. Sprinkle with the optional brown sugar–cinnamon topping for a slight crunch.
Bake: Bake 55–65 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Tent with foil after 45 minutes if the top browns too quickly.
Cool: Let the loaf cool in the pan for 10–15 minutes. Lift out using the parchment and cool completely on a rack before slicing.