1/2 cup (1 stick) unsalted butter, at room temperature, plus more for pan
1 2/3 cups all-purpose flour, plus more for pan
4 medium very ripe bananas (about 13 ounces), peeled and mashed
1/4 cup sour cream
1 teaspoon vanilla extract
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
1 cup (packed) dark brown sugar
2 large eggs, at room temperature
1/2 cup chopped toasted walnuts
A 9x5" loaf pan
Preheat oven to 350°F. Grease a 9x5" loaf pan with butter and dust with flour, tapping out any excess.
Combine banana, sour cream, and vanilla in a medium bowl. Sift together flour, baking powder, baking soda, and salt in a large bowl.
Using an electric mixer, beat butter and brown sugar on medium speed in another large bowl until light and fluffy, about 3 1/2 minutes. Add eggs one at a time and beat on medium speed until fully combined, about 1 minute each. Add banana mixture and beat until just combined, about 30 seconds. Add dry ingredients in 2 batches, beating on low after each addition and scraping down sides of bowl if necessary, until fully incorporated, about 20 seconds per batch. Fold in walnuts. Transfer batter to prepared pan and smooth top with a spatula.
Bake, rotating halfway through, until batter is set, top is dark golden brown and starting to crack, sides are starting to pull away from pan, and a tester inserted into center of bread comes out clean, 60–65 minutes. Cool in pan on a wire rack at least 15 minutes. Run a butter knife around perimeter of pan to loosen loaf, then tap pan gently on its side until loaf releases. Transfer to a cutting board or plate and let cool completely before slicing.
To quickly ripen unripe bananas for baking, bake them unpeeled on a baking sheet at 250°F until soft, 15-20 minutes. Do ahead: Banana bread can be baked, cooled, then wrapped in plastic and left at room temperature up to 3 days, or frozen up to 3 months.