In a large bowl combine the unsalted butter, vanilla, salt, and flour. Use a fork or a pastry cutter to combine together until a crumbly texture has formed and the butter is evenly distributed throughout.
Press the mixture into the bottom of your lined baking dish to make a crust and bake for 10 minutes. Allow crust to cool.
FILLING
Peel (if desired) and slice the apples into thin slices. In a large bowl, combine the sliced apples, sugar, flour, lemon juice, and apple pie spice.
Layer the apple filling over the crust. Smooth out the apples so they lay flat and form an even layer.
TOPPING
In a large mixing bowl, combine the sugar, brown sugar, flour, salt, cinnamon, and butter. Use a pastry cutter or fork to combine until a crumbly mixture forms.
Sprinkle the crumble streusel mixture over top of the apple layer.
Bake for 30-35 minutes.
Allow the bars to cool and then chill in the refrigerator for 1-2 hours. Cut into squares and serve.
Notes
Storage: Store leftover apple crumb bars in an airtight container at room temperature for up to two days. For longer storage, keep them in the fridge for up to five days. You can also freeze the bars for up to three months; just wrap them tightly or put them in a freezer bag. Thaw before serving.