Preheat your oven to 350°F (175°C). Line a standard loaf pan with parchment paper, greasing the sides.

Melt the unsalted butter in a nonstick saucepan over medium heat until golden brown, about 4-5 minutes.

Combine both granulated and light brown sugars in the warm butter, stirring until mostly dissolved, about 1-2 minutes.

Cool the sugar mixture for 3-5 minutes, then add the eggs and vanilla extract, mixing until creamy.

Mash the overripe bananas in a separate bowl, then fold them into the butter-sugar mixture.

Sift together the flour, baking soda, and salt in another bowl, then add it to the wet ingredients gradually, folding gently.

Fold in the sour cream or chosen substitute until just mixed.

Pour the batter into the prepared loaf pan and top with the cinnamon sugar mixture.

Bake for approximately 50 minutes, until a toothpick comes out with a few moist crumbs.

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.