Prep the pan and oven: Heat oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.

Make the crumb: In a bowl, mix 1 cup flour, 1/2 cup brown sugar, 1 teaspoon cinnamon, and 1/4 teaspoon salt. Cut in 6 tablespoons cold butter (cubed) with a fork or fingers until clumps form. Chill while you make the batter.

Mash bananas: In a large bowl, mash bananas until mostly smooth with a few small lumps. You want about 1 1/2 cups.

Whisk wet ingredients: Add melted, cooled 1/2 cup butter, 2 tablespoons oil, 1/2 cup brown sugar, 1/4 cup granulated sugar, 2 eggs, 2 teaspoons vanilla, and 1/2 cup yogurt or sour cream. Whisk until combined.

Combine dry ingredients: In another bowl, whisk 2 cups flour, 1 teaspoon baking soda, 1 teaspoon baking powder, 1/2 teaspoon salt, and 1/2 teaspoon cinnamon.

Make the batter: Add dry ingredients to wet. Stir gently with a spatula just until no dry streaks remain. Do not overmix.

Layer and top: Spread half the batter into the pan. Sprinkle about one-third of the crumb over it. Add the remaining batter, then cover evenly with the rest of the crumb, pressing lightly so it adheres.

Bake: Bake 55–70 minutes, until a tester comes out with moist crumbs but no wet batter. If the top browns too fast, tent loosely with foil for the last 15 minutes.

Cool: Let the loaf cool in the pan 15 minutes, then lift out and cool on a rack until just warm.

Glaze: Stir 1/2 cup powdered sugar with 1–2 tablespoons milk and 1/2 teaspoon vanilla until thick but pourable. Drizzle over the cooled loaf. Slice with a serrated knife.