Heat the oven to 350°F (175°C). Grease a 9 x 5 inch loaf pan and line it with a sling of parchment so you can lift the loaf out easily.
Toast the nuts. Spread walnuts or pecans on a baking sheet and toast in the warming oven for 6 to 8 minutes until fragrant. Cool, then chop. This tiny step adds a deep, toasty flavor that makes every slice better.
Mash the bananas in a mixing bowl with a fork until mostly smooth with a few small bits. You need about 1 1/3 cups. If your bananas were frozen, thaw and lightly drain excess liquid (keep a tablespoon or two for flavor).
Whisk in the melted butter, eggs, sugar, yogurt, and vanilla until the mixture is glossy and combined. It will smell like banana custard. Good sign.
In a separate bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon. This keeps the leavening even so you don’t get pockets of baking soda.
Tip the dry ingredients into the wet. Use a spatula to fold gently until you no longer see dry streaks. Stop when it’s combined. A few lumps are okay. Overmixing is the enemy of tender bread.
Fold in the toasted nuts (and any optional add-ins). Let the batter sit for 5 minutes. This quick rest helps the flour hydrate and the loaf rise more evenly.
Pour into the prepared loaf pan and smooth the top. For a rustic split down the middle, run a lightly oiled butter knife along the center.
Bake 55 to 65 minutes. The top should be deep golden and a tester inserted in the center comes out with a few moist crumbs, not wet batter. If the top is browning too fast around minute 45, tent with foil. For extra certainty, the internal temperature should hit about 200°F (93°C).
Cool in the pan for 10 to 15 minutes, then lift the loaf onto a rack and cool at least 45 minutes before slicing. I know, it’s tough. Warm slices tend to crumble, so give it a little time to set.