1. Sift the flour and salt into a bowl. Beat the egg yolks and milk into the flour.
2. Arrange the apples in a buttered 1.1 -litre (2-pint) ovenproof pie dish. Whisk together the butter and sugar and add cinnamon, then stuff the mixture into the apples.
3. Whisk the egg whites to soft peaks and fold into the batter. Pour the mixture over the apples and bake in the oven at 180°C (350°f) mark 4 for about 1 1/4 hours until the apples are tender and the batter well risen and golden. Serve immediately, dusted with icing sugar.