Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line with parchment. Whisk flour, baking soda, and salt together in a bowl.

Mash bananas in a large bowl. Mix in melted butter, granulated sugar, egg, and vanilla until glossy.

In a small bowl, stir brown sugar and cinnamon together. Add melted butter and mix until it looks like damp sand.

Fold the flour mixture into the banana mixture with a spatula just until no dry streaks remain.

Spoon half the batter into the pan. Dollop half the cinnamon paste over it. Add remaining batter and remaining paste. Swirl with a knife in a figure-eight motion, then bake 50 to 55 minutes until a toothpick comes out with moist crumbs.

Cool in the pan 10 minutes, then turn out onto a rack and cool completely. Slice thickly with a serrated knife.