Use a potato masher to mash the butter, sugar, flour and bicarbonate of soda with the egg, bananas and essence together in a bowl until the mixture has no lumps and is well mixed. Cut each square of chocolate into four then mix into the cake mixture with enough milk to make a dropping consistency. Spread the mixture in the prepared tin and bake in a moderate oven (180 °C / 350°F / Gas Mark 4) for about 50 minutes until cooked through. Turn out and cool on a wire tray.
Cut a 5cm (2 inch) wide slice off one short side of the cake. Assemble the cake on the board, propping up the cut slice with the Flake so that it stands at an angle to one of the short ends. Whisk the dessert topping with the milk until it holds its shape. Place two spoonfuls in the piping bag fitted with the plain writing pipe; add the drinking chocolate to the remainder, stirring it in evenly. Spread the chocolate mixture evenly all over the cake, including the sides. Pipe the figures and signs of a calculator on the Buttons with the plain topping mixture and place them in position. Pipe a plain white line round the edge to complete the cake.