Whisk the butter or margarine, sugar, flour, eggs and vanilla extract in a mixing bowl until light and creamy.
Line a 12-section cake baking sheet with paper cases. Divide the mixture evenly among the cases and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–20 minutes until risen and just firm to the touch. Transfer to a wire rack to cool.
Melt the white, milk and plain dark chocolate in 3 separate bowls, each with one-third of the butter added, over pans of simmering water (see
Put 2 tablespoons each of the melted milk chocolate and plain dark chocolate in separate piping bags fitted with writing nozzles. Spread the milk chocolate over 4 more of the cakes and pipe dots of plain dark chocolate over the milk chocolate. Spread the plain dark chocolate over the 4 remaining cakes and scribble with lines of piped milk chocolate.