Preheat oven to 350 degrees. Line an 8 by 8 baking dish with parchment paper, and let the sides overhang so that you can lift the bars out of the pan once cooked.

First, make the shortbread crust. In a bowl, beat together the softened butter and sugar. Add the vanilla and flour and mix to combine

Press the dough into the bottom of the lined pan

Bake for 15 minutes at 350 degrees

While the crust is baking, prepare the filling and crisp bar topping

For the filling, stir together the thinly sliced apples, lemon juice, sugar, cinnamon, and flour in a bowl

To make the crisp topping, mix together the brown sugar, salt, and flour in a bowl. Use a pastry cutter or knives to cut the cold butter chunks into the flour mixture until crumbly.

Once the crust is done baking, remove it from the oven and place the apple filling evenly over the crust. 

Distribute the crisp topping over the apple filling

Bake at 350 degrees for 45 - 55 minutes or until the top is golden brown and bubbly. 

Allow the bars to cool in the pan for at least 30 - 45 minutes, then gently use the parchment paper to pull the bars out of the pan. Cut into squares and serve