1. Preheat oven to 350°F. Line an 8x8 metal baking pan with parchment paper, leaving an overhang on the sides to use as handles. Lightly coat with cooking spray.
2. In a large mixing bowl, beat the softened butter and brown sugar with a mixer until light and fluffy, about 2-3 minutes. Mix in the vanilla extract.
3. Add the flour and mix on low speed until the dough is soft and crumbly.
4. Press the dough firmly and evenly into the bottom of the prepared pan. Bake for 12-14 minutes, until the crust is slightly puffed. Set aside.
5. In a separate large bowl, gently toss the apple slices with the flour and granulated sugar until evenly coated. Spread this mixture over the pre-baked crust.
6. To make the topping, whisk together the rolled oats, flour, brown sugar, granulated sugar, cinnamon, and salt in a medium bowl.
7. Add the cold, cubed butter. Use a pastry cutter or your fingertips to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with pea-sized pieces of butter remaining.
8. Sprinkle the oat topping evenly over the apple layer.
9. Bake uncovered for 45 minutes, until the topping is golden brown and the apple filling is bubbling at the edges.
10. Let the bars cool completely in the pan on a wire rack. For best results and the cleanest cuts, chill the cooled bars in the refrigerator for at least 1 hour before slicing.
11. Use the parchment handles to lift the entire block out of the pan, place it on a cutting board, and slice into 12 bars.
