½ cup (1 stick) butter, plus a little extra for the pan
3 medium bananas, very ripe
¾ cup buttermilk
¾ cup (packed) light brown sugar
2 large eggs
1 teaspoon pure vanilla extract
1½ cups unbleached all-purpose flour
½ teaspoon salt
1½ teaspoons baking powder
½ teaspoon baking soda
Step 1 – Prior to starting, open your butter and slice into cubes and let soften.
Step 2 – Adjust the oven rack to the center position and preheat the oven to 350°F (325°F if you’re using a glass pan). Put a little soft butter on a paper towel, a piece of waxed paper, or a butter wrapper, and lightly grease the bottom and corners of a standard-sized loaf pan. (No need to grease the sides.)
Step 3 – Peel the bananas, and place them in a medium-sized bowl. Mash with a fork until they form a mostly smooth pulp; then switch from mashing to mixing as you drizzle the buttermilk directly into the bananas. (You can use the fork or a whisk.) Keep mixing until the mixture is completely blended, and then set this aside.
Step 4 – Crumble the brown sugar into the bowl with the softened butter. Use a handheld electric mixer at high speed (or if you don’t have one, use a whisk, with enthusiasm) to beat the butter and sugar together until light and fluffy. Use a rubber spatula to scrape the sides of the bowl a few times during this process. Add the eggs, one at a time, beating well after each addition. Add the vanilla extract, and continue to beat for another minute or two, until everything is well combined.
Step 5 – Combine the flour, salt, baking powder, and baking soda in a second medium-sized bowl. Whisk slowly to blend. Add about half of this dry mixture to the butter mixture, stirring it in with a wooden spoon. Then stir in about half of the banana-buttermilk mixture. Repeat with the remaining dry mixture, followed by the remaining banana-buttermilk mixture, stirring from the bottom of the bowl after each addition, just enough to thoroughly blend without overmixing.
Step 6 – Transfer the batter to the prepared pan, taking care to scrape all of it in with a rubber spatula. Then use the spatula to spread the batter evenly. Bake for 50 to 70 minutes, or until a sharp knife inserted all the way into the center comes out clean.
Step 7 – Remove the pan from the oven and allow the bread to cool in the pan for at least 15 minutes before removing it. (The best way to do this is to rap the pan sharply on the counter a few times to loosen the bread, and then let it slide out onto a cooling rack.) To avoid crumbling, wait at least 20 minutes longer to slice and serve.