In a medium saucepan, melt 4 tablespoons of unsalted butter over low heat. Add 1 cup of brown sugar, stirring until dissolved and smooth, about 2-3 minutes. Once mixed, remove the pan from heat and pour the caramel mixture evenly into a greased 9-inch round cake pan. Arrange sliced ripe bananas decoratively over the caramel, making sure they overlap slightly for an attractive topping.

In a large mixing bowl, cream together ½ cup of unsalted butter and ¾ cup of brown sugar until light and fluffy, about 3-4 minutes with an electric mixer. Add 2 large eggs, one at a time, mixing well after each addition. Stir in ½ cup of milk and 1 teaspoon of vanilla extract until fully combined, creating a rich and smooth batter.

In a separate bowl, whisk together 1 ½ cups of all-purpose flour, ½ cup of cocoa powder, and 1 teaspoon of baking powder. Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Preheat your oven to 350°F (175°C). Carefully pour the cake batter over the arranged bananas in the pan, smoothing the top with a spatula. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes. Place a large plate over the pan and quickly invert it, allowing the cake to release onto the plate.