Preheat the oven to 190 C / Gas 5. Line a muffin tray with cupcake cases.
Beat the softened butter or margarine with the caster sugar in a bowl until the mixture is light and fluffy. Add the flour, eggs, milk and lemon rind to the bowl and mix until smooth.
Divide the mixture between the paper cases and bake in the centre of the oven for 10-15 minutes, until the cakes have risen and an inserted skewer come out clean.
Remove from tray and place on cooling rack. Immediately sprinkle each cupcake with a generous pinch of granulated sugar. Pierce each cake with a cocktail stick three times on the top and pour over a teaspoon of lemon juice.