Coat a casserole dish with cooking spray and lay apples in it.
Mix together brown sugar, margarine, and cinnamon. Add in egg and vanilla. Combine flour with baking powder and salt and mix into brown sugar mixture. Spoon batter over top of apple slices.
To make the brown sugar syrup, add the 1/2 cup brown sugar, 2 tbsp flour, and 1 tsp margarine in a pot. Mix in water and bring to a boil, stirring continually. Continue boiling 2 to 3 minutes, stirring continually. Take pot off of heat and mix in the 1/2 tsp vanilla extract.
Drizzle Brown Sugar Syrup over batter. Bake at 375 degrees F for 50 to 55 minutes or until apples are soft. Serve warm.