Caramelize the butter and sugar to a light color at the bottom of a 10 diameter baking form. Spread out the caramel evenly.
During baking of the cake, it will continue to caramelize. Arrange the bananas on top, cut into round slices.
Melt the butter for the dough and leave it to cool. To it add the sugar and egg and beat with a mixer until you get a homogeneous creamy consistency. Add the flour, milk and rum (essence), the pinch of salt and baking powder and beat once again.
Pour 2/3 of the dough over the bananas. To the rest of it, add the cocoa with a spatula and stir well. Pour this over the white dough and use a fork to in order to make stripes on the surface.
Bake at 356°F (180 °C) for 40-45 min. or until a toothpick comes out dry when you poke the cake. Let it sit for 5 min., then invert it onto a platter.
Optionally, pour in a cup of water to the remaining caramel at the bottom of the baking form, heat it briefly for the caramel to melt and syrup the cake with it.