Start by making the filling. Put the first four ingredients for the curd in a bowl and whisk together.
Then add the butter, cut into lumps, and fit the bowl over a pan of simmering water. Now take a mini-whisk and whisk from time to time until the curd thickens – about 10 minutes.
Then remove the bowl and leave the curd to get quite cold.
To make the cakes, combine all the ingredients together in a bowl and, using an electric whisk, beat until absolutely smooth (1–2 minutes).
Then drop an equal quantity of the mixture into the paper cases and bake near the centre of the oven for 15–20 minutes or until the cakes are well-risen and golden.
Then remove them to a wire rack and leave to cool.
To fill the cakes, angle a small sharp knife and cut to within about 1cm of the edge of each cake to remove a cone-shaped round, leaving a cavity in the centre.
Cut the round in half and set aside. Now fill the centre of each cake with the curd and return the two pieces of cake to sit on top like butterfly wings.
Finally dust the cakes with sifted icing sugar and store in an airtight tin
Pre-heat the oven to 190ºC, gas mark 5