Put the eggs, sugar, water and essence into a bowl and put them over a saucepan which has been half-filled with boiling water. Whisk the mixture well over this heat for five minutes. Put the saucepan over heat, and continue whisking for five minutes. Take the bowl from the saucepan and whisk until the mixture is cold and thick. Fold in the sifted flour very carefully. Pour into an eight-inch greased and lined cake tin, and bake at 400°F, 200°C, gas mark 6 for twenty-five minutes. Cool on a wire cake tray. When the cake is cold, split with a sharp knife and fill with jam or lemon curd. Dust the top with icing sugar.