Position a rack in the center and heat the oven to 350°F. Grease a 9x5-inch loaf pan with butter or nonstick spray and line it with a parchment sling (two long strips crossing the bottom and hanging over the sides) — this is what lets the loaf lift out cleanly later.

In a large mixing bowl, mash the bananas with a fork until mostly smooth with a few small lumps left — you should have about 1 1/4 cups. Deeply spotted or black-peel bananas are the point here; they bring the most moisture and banana flavor.

Whisk the melted butter, brown sugar, and granulated sugar into the mashed bananas until smooth. Add the eggs one at a time, whisking well after each, then whisk in the sour cream and vanilla until the mixture is completely combined and glossy. The sour cream is the moisture secret: its fat and acidity keep the crumb tender for days.

In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.

Pour the dry ingredients into the wet bowl and fold gently with a rubber spatula just until no dry streaks remain. A few small lumps are fine. Overmixing develops gluten and turns banana bread dense and tough, so stop as soon as the flour disappears.

Spread the batter evenly into the prepared 9x5-inch pan and smooth the top. Bake at 350°F for 60–65 minutes, until deep golden with a cracked dome. If the top browns too quickly, loosely tent with foil after about 40 minutes. The loaf is done when a toothpick inserted in the center comes out with a few moist crumbs — not wet batter.

Cool in the pan on a wire rack for 15 minutes, then lift out with the parchment sling and cool completely on the rack before slicing — at least 1 hour. Slicing too early makes the moist crumb gummy. Makes 10 slices.