Preheat your oven to 350°F (180°C) and grease a 9"x5" loaf pan, lining it with parchment paper.

Melt the unsalted butter in a saucepan over medium heat until browned, then transfer to a mixing bowl.

In a separate bowl, whisk together all-purpose flour, baking soda, baking powder, and kosher salt.

Whisk granulated sugar into the cooled brown butter, then add mashed bananas, sour cream, eggs, and whisk until smooth.

Fold the dry ingredients into the wet mixture until no flour streaks remain.

Scoop out a third of the batter into a bowl, sift in matcha powder, and stir until fully incorporated.

In the prepared loaf pan, alternate spoonfuls of banana batter and matcha batter, gently swirling with a knife.

Bake for 55-60 minutes until golden and a toothpick comes out clean.

Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.