Heat the oven to 175 C / 350 F. Line an 8x8 inch baking pan with parchment, leaving an overhang on two sides.

Melt 15 g butter in a 10-inch nonstick skillet over medium heat. Add the banana slices in a single layer and sprinkle the 30 g brown sugar over them.

Cook undisturbed for 90 seconds, then flip each slice and cook for another 60 to 90 seconds until the cut sides are deep amber and the pan smells like toffee.

Transfer the caramelized bananas and all the sticky pan juices to a bowl. Mash roughly with a fork, leaving a few small chunks. Set aside to cool for 5 minutes.

In a large mixing bowl, whisk together the melted 90 g butter and 130 g brown sugar until combined and slightly glossy.

Add the eggs one at a time, whisking well after each. Stir in the sour cream and vanilla extract until smooth.

Fold in the mashed caramelized bananas with a rubber spatula until evenly distributed.

Add the flour, baking powder, baking soda, and salt directly to the bowl. Fold gently until no dry streaks remain - do not overmix.

Pour the batter into the prepared pan and spread to the edges. Tap the pan once on the counter to release any air pockets.

Bake for 28 to 32 minutes, until the top is golden brown and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.

Rest the cake in the pan for 15 minutes before lifting out using the parchment overhang. Slice into 9 squares and serve warm or at room temperature.