Preheat your oven to 350°F (175°C) and grease your 8-inch cake pan thoroughly with butter or non-stick spray.
In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt until no lumps remain.
In a separate bowl, cream the softened butter and granulated sugar together until the mixture is pale and fluffy, about 3 minutes.
Beat in the egg and vanilla extract until fully incorporated, then stir in half of the mashed bananas.
Gradually alternate adding the dry ingredients and the milk into the butter mixture, beginning and ending with the dry ingredients, mixing until just combined.
Pour the batter into the prepared pan and bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs.
Allow the cake to cool completely on a wire rack before attempting to add the topping.
While the cake cools, whip the heavy cream and powdered sugar in a clean bowl until stiff peaks form.
Gently fold the remaining mashed bananas and the cooled, melted chocolate into the whipped cream until the mousse is uniform in color.
Spread the banana chocolate mousse evenly over the top of the cooled cake and refrigerate for at least 30 minutes before serving.