Grease and flour a baking dish. Cream together 20g softened butter and 70g caster sugar, then beat in 3 egg yolks and
the zest of a lemon. Stir in 30g plain flour, 250ml whole milk and the juice of the lemon. Whisk 3 egg whites and another
70g caster sugar until stiff and fold into the batter. Tip into the greased baking dish, place the dish in a larger
high-sided pan, half fill with hot water (this is a bain marie!) and bake in a 180°C oven for 25-30 minutes,
until golden brown on top. Let cool until warm and serve dusted with icing sugar.