Prep your pan and oven. Heat the oven to 350°F (175°C).
Grease a 9x5-inch loaf pan and line it with a parchment sling so you can lift the bread out easily.
Mash the bananas. In a large bowl, mash the ripe bananas with a fork until mostly smooth with a few small lumps. You should have about 1 1/4 to 1 1/2 cups of mashed banana.
Mix the wet ingredients. Whisk in the melted butter, sugar, eggs, and vanilla until the mixture looks glossy and combined. Stir in the yogurt or sour cream.
Combine the dry ingredients. In a separate bowl, whisk the flour, baking soda, salt, and cinnamon.
This helps distribute the leavening evenly.
Bring it together. Add the dry ingredients to the wet. Gently fold with a spatula just until no streaks of flour remain. If using add-ins, fold them in now. Do not overmix—a few small lumps are fine.
Fill the pan. Scrape the batter into the prepared loaf pan and smooth the top.
For a nice crack, you can run a thin line of melted butter down the center.
Bake. Bake for 55–65 minutes, or until the top is deep golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs. If the top browns too fast, tent loosely with foil during the last 15 minutes.
Cool. Let the loaf rest in the pan for 10–15 minutes. Lift it out and cool on a rack for at least 45 minutes before slicing.
This helps it set and stay moist.
Slice and enjoy. Use a sharp serrated knife for neat slices. Serve plain, with butter, or toasted.