Preheat oven to 300°F. Line 8×8 baking pan with parchment.
Bring parchment up the sides for easy lifting later.
Cream butter on high in stand mixer for 5 minutes.
Add sugar, vanilla, salt to butter, and mix until combined.
Incorporate flour and stir well until mixture forms dough.
Press dough into parchment-lined pan, bake for 15 minutes.
Combine apples, flour, sugar, cinnamon, nutmeg in mixing bowl.
Mix flour and ¾ cup brown sugar. Cut in butter.
Remove crust, increase oven temperature to 350°F immediately.
Sprinkle half topping on crust, layer apples evenly on top.
Keep layers level; sprinkle remaining topping evenly over apples.
Bake in oven at 350°F for 30 minutes until golden.
Allow bars to cool completely before placing in refrigerator.
Set in fridge for at least 2 hours for firmness.
Remove by lifting parchment; cut into 16 squares, and serve.
