Heat oven to 150C/300F. Butter and line a 23cm cake tin.
Coat the apples in lemon juice and set aside. In a large bowl beat the butter and sugar until pale and fluffy, add the eggs and beat until smooth. Sift in the flour, baking powder ans 1/2 tsp salt. Mix until incorporated. Slowly add the milk, mixing well after each addition until you have a smooth batter.
Transfer the batter to the tin and arrange the apples in a spiral over the batter. Mix the sugar and cinnamon and sprinkle on top. Bake for 45-50 mins until a skewer comes out clean and the top is golden.
Remove and cool for 15 mins in the tin. Serve warm or at room temperature with whipped cream.