2) Beat the egg yolks until foamy and add the buttermilk. Add the flour mix to the egg mix, combining thoroughly to ensure that they are well incorporated.
3) Add the lemon juice to the grated apple. Pour the batter into the apple mixture. Whip the egg whites to stiff peaks, and add sugar. Gently fold into the apple batter.
4) Melt the butter in a ovenproof non-stick frying pan. Pour in the batter and sprinkle with the cinnamon and sugar. Toss on the chopped walnuts and bake in the oven for 20 minutes. Remove and dust with icing sugar. Cut into wedges and serve immediately.