Preheat the oven to 350°F (175°C) and grease or line a 9x5-inch loaf pan with parchment paper.

In a small bowl, mix 50 g granulated sugar, 2 tsp cinnamon, and 20 g melted butter to make the cinnamon swirl mixture, then set aside.

In a large mixing bowl, mash the bananas with a fork until mostly smooth.

Add 80 g granulated sugar, 80 g brown sugar, 75 g melted butter, eggs, and vanilla to the bananas and whisk until well combined.

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

Add the dry ingredients to the wet ingredients and gently fold with a spatula until just combined and a few streaks of flour remain.

Fold in the yogurt or sour cream just until the batter is smooth and no dry spots remain, taking care not to overmix.

Pour half of the batter into the prepared loaf pan and spread it into an even layer.

Drizzle or spoon half of the cinnamon swirl mixture over the batter, then use a knife to gently marble it.

Pour the remaining batter on top and spread evenly, then add the remaining cinnamon mixture and marble again lightly.

Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Let the banana bread cool in the pan on a cooling rack for 15 minutes, then lift out or turn out and cool for at least 5 more minutes before slicing.