Preheat the oven to 180C/350F/Gas 4 (160C fan).
Put all the cake ingredients into a large bowl and beat well for 2-3 minutes, or until the mixture is well-blended and smooth. Fill each paper case with about 35g/1¼oz of mixture.
Bake in the preheated oven for about 15-20 minutes, or until the cakes are well risen and golden-brown. Lift the paper cases out of the bun tin and cool the cakes on a wire rack.
When cool, cut a disc from the top of each cake leaving a little gap around the edge and cut this slice in half. Spoon half a teaspoonful of orange curd in each.
To make the icing, beat the butter and icing sugar together until well blended. Pipe or spoon a swirl of buttercream on top of the orange curd and place the half slices of cake on top to resemble butterfly wings. Dust the cakes with icing sugar to finish.