Preheat oven to 350°. Grease a 9 x 13 baking pan.
For the crust, mix the flour and brown sugar together, then cut in the butter using a pastry blender, two knives or your hands. Press the mixture into bottom of prepared pan. Set aside.
For the topping, mix the dry ingredients together then mix in the melted butter. Stir well until everything is moist. Set aside.
For the filling, mix the dry ingredients together in a small bowl. Peel and core apples, then chop into 1/4" - 1/2" pieces. Pour the sugar mixture over the apples and blend it so all the apples are coated completely.
Place the apples on the crust in the pan, spreading them out to form an even layer. Sprinkle topping over apples, breaking up clumps as you go. Cover the entire surface of the apples, then press topping down gently with the palms of your hands. Bake for 45 minutes. Cool completely before cutting into the bars.