Preheat the oven to 190 C (gas mark 5). For the cupcakes, cream the butter and sugar together until smooth. Whisk the eggs separately in a bowl, then add to the mixture along with the flour, milk and lemon rind and beat together until smooth.
Divide the mixture between the paper cases and bake in the oven for 14-16 minutes until firm to the touch. Remove from the oven and transfer to a wire rack to cool.
Immediately sprinkle the sugar over the cakes, then spoon over the lemon juice and leave to cool.