Preheat oven to 375°F. Grease a 9x13 inch baking dish.
Cream sugar and butter together; add egg and mix well.
In a separate bowl, blend flour, salt and baking powder together. Add to creamed mixture.
Mix baking powder with milk and vanilla; combine all ingredients together.
Fold in apple slices and pour batter into prepared baking dish.
In a separate bowl, combine brown sugar, flour, cinnamon and butter, mixing until crumb texture forms. Sprinkle over apple batter.
Bake at 375 degrees for 35 to 40 minutes, until golden brown and set.
