First make the cake. Base line 2 × 20cm, deep cake tins with baking paper. Heat the oven to 180C/fan 160C/gas 4. Beat the sugar and eggs with an electric whisk for 10 minutes, until pale and fluffy, and stiff enough to leave a trail when you lift the beaters. Sift over both flours and fold in. Divide between the tins. Bake for 20 minutes, until golden and a skewer pushed into the cake comes out clean. Cool for 5 minutes in the tin (they will sink a little but don’t worry) then turn out onto a wire rack. Wash and dry one of the cake tins, and line it with a double layer of clingfilm or greaseproof paper.