Use 2 × 20-cm straight-sided sandwich tins. Grease and line the bases with baking paper. Preheat oven to 190°C.
Beat eggs and sugar in a large heatproof bowl over a saucepan of simmering water until thick and creamy. Sift ﬂour and baking powder into the bowl and, using a metal spoon, carefully fold it into the creamed mixture. Gently fold in the cooled melted butter.
Divide mixture equally between the 2 tins and spread evenly. Bake for 20 minutes or until cakes are well risen, springy to the touch and have shrunk slightly from the side of the tins. Cool for 5 minutes. Turn out on a wire rack and leave to cool completely.
Place one cake upside-down on a serving plate and spread evenly with the jam. Whisk cream until it is just thick enough to hold its shape and spread evenly over the jam, just up to the edge of the sponge. Place second cake on top of the ﬁlling and sift caster sugar evenly over the top.