Preheat & Prepare Pan: Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan.
Make the Caramelized Banana Topping: In a saucepan, melt butter and brown sugar over medium heat, stirring until smooth. Pour into the cake pan and arrange banana slices over the caramel.
Prepare the Cake Batter: In a bowl, whisk flour, baking powder, baking soda, and salt.
Cream Butter & Sugar: In a separate bowl, beat butter and sugar until fluffy. Add eggs one at a time, then mix in vanilla and mashed banana.
Combine & Mix: Alternate adding the dry ingredients and buttermilk to the batter, mixing until just combined.
Bake: Pour batter over the banana topping. Bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Flip & Serve: Let the cake cool for 10 minutes, then invert onto a plate. Serve warm and enjoy!