Preheat the oven to 170°C, and line a cake tin with baking paper.
Whisk together the egg yolks, yoghurt, maple or honey, cornflour, vanilla and baking soda.
In a seperate bowl, beat the egg whites until soft peaks form, then gently fold into the batter, being careful not to overmix.
Pour the batter into a cake tin, and smooth out evenly. Bake for 30 minutes, then turn off the oven, leaving the cake in the oven to cool for at least half an hour. It will deflate in size somewhat.
Carefully transfer to a serving plate, and dust with icing sugar if you desire. Also delicious topped with whipped cream, fresh fruit, grated chocolate or lemon curd.
