Preheat the oven to 300 degrees F. Line your 8×8 baking pan with parchment paper. You want this to cover the bottom as well as come up the sides.
Start by making your shortbread crust. Place 1/2 cup of softened butter into a mixing bowl. Mix with a mixer on high for 5 minutes. You want to cream the butter until it turns white in color. Add in the 1/4 cup sugar, vanilla, and salt. Mix well then add in 1 cup of all purpose flour. Press this mixture into your lined pan. Bake in the oven for 15 minutes.
While your crust is baking, start on your filling. Peel, core, and slice your apples. Add a little bit of lemon juice to the apples to prevent browning. Add in 2 tablespoons all-purpose flour, 2 tablespoons sugar, cinnamon, and nutmeg. Mix until apples are coated.
Mix your topping next by adding 1/2 cup flour and 3/4 cup brown sugar to a bowl. Cut in 1/4 cup butter with a pastry cutter until the mixture is crumbly.
When crust is done baking, remove from oven and turn up the heat to 350 degrees F. Sprinkle about half of the topping mixture over the crust. Then add on your apple filling mixture. Sprinkle the remaining topping mixture on the top of everything. Bake for 30 minutes. Remove from oven and allow to cool. Place in the refrigerator for about an hour before slicing. Use the parchment paper to lift from the pan and cut into squares. Enjoy your apple bar recipe as dessert or a snack anytime of the day!