Preheat oven to 350°F (175°C).

Line a 9x5-inch loaf pan with parchment paper and lightly spray with non-stick cooking spray.

Mash bananas in a large bowl until smooth with some small lumps.

Whisk in melted butter, sugar, eggs, and vanilla until well combined.

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

Gently fold dry ingredients into the wet mixture, alternating with buttermilk or sour cream. Do not overmix.

In a small bowl, mix sugar, brown sugar, cinnamon, melted butter, and flour until it forms a thick paste.

Pour half the banana bread batter into the loaf pan.

Spoon or sprinkle half of the cinnamon mixture over the batter.

Repeat with remaining batter and cinnamon mixture.

Swirl gently with a butter knife to create a marbled effect.

If using, sprinkle cinnamon-sugar topping over the batter.

Bake for 55-65 minutes, until a toothpick comes out clean.

Let cool in the pan for 15 minutes, then transfer to a wire rack to cool for 1 hour.