Heat oven to 350°F. Spray 9-inch square pan with cooking spray. In large bowl, mix flour, 3/4 cup brown sugar, cinnamon and baking soda.
With pastry blender or fork, cut in 3/4 cup butter until mixture resembles coarse crumbs. Stir in apples and egg. Press mixture firmly in pan. Bake 25 to 30 minutes or until puffed in center.
Meanwhile, in 1-quart saucepan, combine all topping ingredients except whipped topping. Cook over medium heat, stirring frequently, until mixture comes to a boil. Boil 1 minute, stirring constantly. Remove from heat.
Remove apple layer from oven. Drizzle hot topping over warm apple layer; spread evenly. Return to oven; bake 4 to 6 minutes longer or until topping is bubbly. Cool completely, about 1 1/2 hours. With serrated knife, cut into squares. Serve with whipped topping.