Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" square baking tin.
In a large bowl, mash the bananas. Then add the butter, both sugars and the egg (ensure the butter has cooled a little so the egg doesn’t cook). Mix until combined.
Add the flour, baking powder, baking soda, cocoa powder and melted milk chocolate. Fold everything together until combined.
Pour into the lined tin and bake for 35-40 minutes, until a skewer inserted comes out clean. Let cool on a wire rack.
Make the frosting in a large bowl, combine the butter, melted chocolate and half the icing sugar - using an electric whisk on high. Gradually add the remaining icing sugar until thick and smooth. Spread over the cake. Add any optional decorations !Slice and serve!