Preheat that oven to 350°F and line an 8-inch baking dish with parchment paper (makes cleanup way easier).
In a bowl, mix together the butter, vanilla, salt, and flour. Use a fork or pastry cutter until it’s all crumbly. Press it into your pan and pop it in the oven for 10 minutes.
Let the crust cool a bit while you prep the apple filling.
Peel (optional) and thinly slice your apples. Toss them in a bowl with the sugar, flour, lemon juice, and apple pie spice. Spread that over your cooled crust in a nice, even layer.
Now for the crumbly topping. Mix all the topping ingredients in a bowl with a fork or pastry cutter until you’ve got a crumbly texture. Sprinkle it all over the apple layer.
Bake for 30–35 minutes until the top is golden and those apples are bubbly and tender.
Let the bars cool down. Chill them in the fridge for 1–2 hours so they slice nicely.