Cut each half of the fresh or frozen 20-23cm sponge into two layers. Use three of the layers unless you want a very tall layer cake, freezing the remaining layer for other use. Mix the sugar and cornflour thoroughly in a medium to large pot. Add the blackcurrants, water and sherry (or orange juice) and stir rapidly and constantly over a medium heat, until the mixture boils and thickens.
Place the pot standing in cold water until the blackcurrant mixture is cool. Spread the blackcurrant mixture over each layer except the top one. Keep the leftover mixture to serve over accompanying ice cream served with the cake. Spread the whipped cream over the berry layers, and assemble the cake. Place the cake within the refrigerator for a minimum of 8 hours ideally up to 36 hours. The cake should become firm and easy to cut. Sprinkle with icing sugar before you serve.
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