Place 16 mini silicone muffin cases on a baking sheet.
Put the butter, sugar, flour, lemon rind and egg in a bowl and beat with a hand-held electric whisk until light and creamy. Divide among the cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes until risen and just firm. Leave in the cases for 2 minutes, then transfer to a wire rack to cool. Drizzle 2 tablespoons of the limoncello over the cakes and leave to cool completely.
Reserve 2 tablespoons of the lemon curd and spread the remainder over the cakes with a palette knife.
Make the icing by putting the unsalted butter, icing sugar, reserved limoncello, reserved lemon curd and the lemon juice in a bowl and beating well until smooth and creamy. Place in a piping bag fitted with a star nozzle and pipe swirls on top of each cake. Serve lightly dusted with icing sugar.