Beat together the egg yolks and sugar
until pale and creamy. Gradually beat in the flour, potato flour and
vanilla essence. Whisk the egg whites until stiff and fold into the
mixture. Spoon the mixture into a greased and lined 20 cm/ 8 in cake
tin (pan) and bake in a preheated oven at 200°C/400°F/gas mark 6 for 5
minutes only. Remove the cake from the oven and make a cross on the
top with a sharp knife, then return to the oven as quickly as possible
and bake for a further 5 minutes. Reduce the oven temperature to
180°C/350°F/gas mark 4 and bake for a further 25 minutes until well
risen and golden brown. Leave to cool, then serve dusted with icing
sugar.