Preheat the oven to 350 degrees F. and grease a 9x13-inch baking dish with butter or non-stick cooking spray.
Make the Crust
Using a fork or the whisk attachment on an electric mixer, combine butter, all purpose flour, and sugar until a crumbly crust forms. It will seem dry, but that's fine!
Press the crust in the greased pan and bake for about 15 minutes until it’s golden brown.
While the crust bakes, prepare the filling.
Make the Filling
In a large nonstick sauté pan, stir together apples, sugar, butter, cornstarch, cinnamon, vanilla extract, and salt. Cook over low heat for about 10 minutes, or until the apples start to soften. **You can add 1 tablespoon of water if your apples start to thicken too much or are sticking to the pan.
Make the Crumble Topping
To a mixing bowl, add melted butter, sugar, cinnamon and flour. Stir well to combine.
Bake
Remove the crust from the oven and pour the apple filling over it. Use an offset spatula or back of a spoon to spread the filling evenly over the crust. Sprinkle crumble topping evenly over the fruit filling.
Bake at 350℉ for 15-20 minutes, or until the top of the crumble bars is golden brown.
Notes
Helpful Tips from Liz
Preparing the apples: You can slice or dice the apples, whichever you prefer. The important thing is to try and keep the sizes consistent so that the fruit bakes evenly.
Is your filling too thick? As the apple bar filling cooks, it may be too thick for your liking. If so, add 1 Tablespoon of water to the pan and stir to combine.
Allow the dessert to cool before slicing: Until it cools a bit, the topping will be crumbly and the filling will be loose. I recommend letting the apple bars cool for at least 15-20 minutes before you attempt to cut it into squares.
StorageAllow the crumble bars to cool completely, then cover the pan tightly with plastic wrap or a sheet of foil. Store in the fridge for up to 5 days.