Preheat the oven to 180ºC, fan 160ºC, gas 4. Line a 12-hole cupcake tray with paper cases.
In a large mixing bowl, beat together 120g of the butter and the sugar until light and fluffy. Gradually add the eggs, until combined. If the mixture looks to be curdling slightly, just add 1 tablespoon of the flour.
Stir in the 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. Cool completely.
To decorate, 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.