Ingredients (Makes 12)
For the topping
Preheat the oven to 180°C, 350°F, Gas 4. Line a deep, 12-hole muffin tin with paper cases.
Pour the milk into a pan and heat gently until just scalding (take care not to boil). Put the grated chocolate and one-third of the sugar into a bowl. Pour over the warm milk and stir until the chocolate has melted and the mixture is smooth. Leave to cool.
Place the butter in a bowl with the remaining sugar and the vanilla essence and, using a wooden spoon, beat until light and fluffy. Gradually beat in the beaten egg. Stir in the chocolate mixture alternately with the flour. Finally, stir in the chocolate chips.
Spoon the mixture into the paper cases until three-quarters full. Bake in the preheated oven for 15-18 minutes, until the cupcakes spring back when pressed in the centre. Remove from the oven and transfer to a wire rack to cool.
For the topping, combine the icing sugar, lemon zest, if using, and lemon juice or water until smooth and spreadable but not runny, adding a little extra water if necessary. Spoon 1 tsp of the icing over each cupcake and perch a Malteser on top of each cake to decorate.