Preheat oven at 350 degrees F.
Combine dry ingredients together in a bowl including flour, baking powder, baking soda and brown sugar, then sift them together so that they can mix well.
Now beat margarine using beater so that it would become fluffy.
After that add vanilla extract and egg whites to the butter and beat again to make smooth paste.
Now gradually add dry mixture to the margarine batter and mix using flat rubber spatula. In the end add apple slices to the batter.
Pour the cake batter into a greased baking pan and bake for at least 40 mins. in preheated oven until become brown from top.
To prepare topping for apple cake, cook the ingredients in a saucepan mentioned for topping until become thick, it would take 8-10 mins.
When the cake becomes ready cool it to room temperature and then pour the prepared caramel sauce as topping.