Preheat the oven to 350°F (175°C).

Grease a 9-inch round cake pan and set aside.

For the caramelized banana topping:

In a small saucepan, melt the butter over medium heat.

Add the brown sugar and stir until dissolved and bubbly.

Remove from heat and pour the caramel mixture into the prepared cake pan.

Arrange the sliced bananas on top of the caramel in a single layer.

For the cake batter:

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the eggs, one at a time, followed by the vanilla extract.

Gradually mix in the dry ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients.

Pour the cake batter over the caramel and bananas in the cake pan.

Smooth the top with a spatula.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and let cool in the pan for 10 minutes.

Carefully invert the cake onto a serving plate, allowing the caramel and bananas to fall onto the cake.

Serve warm or at room temperature.