In a large bowl, beat together the butter, sugar, flour, eggs, milk and 1 teaspoon of the lemon rind until pale and creamy.
Divide the mixture between the greased or paper case-lined cups of a 12-cup muffin tin and bake in a preheated oven, 190˚C (375˚F), Gas Mark 5, for 12–15 minutes until golden and risen. Cool on a wire rack.
Meanwhile, mix the icing sugar with the remaining lemon rind and just enough lemon juice to create a very thick, smooth icing. Spread over the cold cupcakes and sprinkle with popping candy to serve.