Put the cake mix in a bowl, add the butter, egg and water and mix in. Spoon the cake mixture into the paper cases, place on a baking tray. Press one piece of banana into each cake.
Bake for approx. 20 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool on a rack, remove the muffins from the tin.
Place the couverture in a thin-sided bowl and suspend over the gently simmering bain-marie. The bowl must not touch the water. Melt the chocolate and stir until smooth, stir in the cream cheese and icing sugar. Transfer the mixture to a piping bag with a serrated nozzle (approx. 12 mm diameter) and pipe onto the muffins.