Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan with 1 tablespoon butter or nonstick spray, then dust lightly with 1 tablespoon flour, tapping out excess; alternatively line the pan with parchment, leaving an overhang for easy removal.

Batter

Mash bananas

In a large mixing bowl, peel and mash 3 medium ripe bananas until mostly smooth with a few small lumps remain (about 1 1/2 cups mashed).

Combine wet ingredients

To the mashed bananas add 3/4 cup granulated sugar and 1/4 cup packed light brown sugar; stir until sugar begins to incorporate. Whisk in 2 large eggs until combined. Add 1/2 cup melted and cooled unsalted butter, 1 teaspoon vanilla extract and 1/4 cup buttermilk (or plain yogurt). Mix until homogeneous; do not overwork.

Mix dry ingredients

In a separate medium bowl whisk together 1 3/4 cups all-purpose flour, 1 teaspoon baking soda, 1/2 teaspoon baking powder, 1/2 teaspoon fine salt and 1/2 teaspoon ground cinnamon (if using) until evenly distributed.

Combine wet and dry

Make a well in the wet mixture and add the dry ingredients. Fold gently with a spatula until just combined and no large streaks of flour remain; a few small streaks are fine. Do not overmix to keep the bread tender.

Add-ins

Fold in 1/2 cup chopped walnuts or pecans if using. If batter seems very stiff, add an extra tablespoon of buttermilk to reach a thick, pourable consistency.

Transfer to pan and top

Scrape the batter into the prepared 9x5-inch loaf pan and smooth the top with an offset spatula. If desired, arrange 1/2 cup thin banana slices on top and sprinkle evenly with 1 tablespoon turbinado or coarse sugar for a crunchy crust.

Bake

Bake in the preheated 350°F (175°C) oven for 50–65 minutes, rotating the pan halfway through baking. Start checking at 50 minutes: a toothpick inserted in the center should come out with a few moist crumbs but no wet batter. If the top is browning too quickly, tent with foil.

Cool and serve

Remove the loaf from the oven and let it cool in the pan on a wire rack for 10–15 minutes. Run a knife around the edges, lift the bread out using the parchment overhang (or invert if greased), and cool completely on the rack before slicing to avoid a gummy crumb. Serve at room temperature; store wrapped at room temperature for up to 3 days or freeze slices for longer storage.