Line a 12-section bun tray with paper cake cases. Put the butter, sugar and vanilla extract in a bowl and beat with a wooden spoon until light and creamy. Add the eggs and beat the mixture again, then sift in the flour and cocoa powder and stir in. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–15 minutes or until risen and firm to the touch. Transfer to a wire rack to cool.
Put about three-quarters of the icing in a bowl, add the water and stir until you have a thick but spreadable icing. When the cakes are cool, spread the icing over the tops with the back of a teaspoon or with a palette knife.
Roll small pieces of the remaining icing into devil's horns and then position them in the wet icing on top of the cakes. Leave to set.