01 -
Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square cake pan.
02 -
Spread brown sugar on the bottom of the prepared pan and scatter butter pieces on top. Place the pan in the preheated oven until butter has melted, 5 to 7 minutes. Remove from the oven; stir and spread mixture evenly on the bottom of the pan. Arrange sliced bananas to cover the bottom. Set aside.
03 -
Stir together flour, baking powder, baking soda, cinnamon, and salt in a large bowl; set aside.
04 -
Beat together white sugar and butter with an electric mixer in a large bowl. Mix in eggs and vanilla until well combined. Add mashed bananas and sour cream; mix to combine.
05 -
Add the dry ingredients to the wet ingredients on low speed and beat until batter is just combined.
06 -
Pour the batter over the banana slices in the pan and smooth the top. Bake in the preheated oven until a toothpick inserted into the center comes out completely clean, 30 to 35 minutes.
07 -
Let the cake cool in the pan for about 30 minutes before inverting and slicing.