Preheat the oven to 170°C/150°C fan/gas 3. Grease a 22cm-diameter cake tin with butter and line the bottom with baking paper. In a bowl, beat the egg whites 2 tbsp water until stiff. Gradually beat in 50g sugar and the vanilla extract. Stir in the egg yolks, then fold in the flour, cornflour and baking powder.