Preheat the oven to 180ºC/gas mark 4. Line a 12-hole cupcake tray with paper cases.
In a large mixing bowl, beat together 120g butter and the sugar until light and fluffy. Gradually add the eggs, until combined. If the mixture starts to curdle, add a little flour.
Stir in the lemon zest and sift the flour into the mixture. Fold together until you have a smooth batter. Divide the mixture among the paper cases. Bake for 20-25 minutes until risen, then cool completely.
Meanwhile, beat the remaining butter, lemon juice and icing sugar together until smooth. Slice a circle off the top of each cake and fill the cavities with a little lemon curd and a swirl of the lemon icing. Cut each sliced round in half and arrange on top of the filling to resemble butterfly wings. Dust lightly with icing sugar.