½cupunsalted butter, cold and cut into small cubes
Preheat oven to 300°F. Line the bottom and sides of a 9" x 13" baking pan with parchment paper. Set aside.
Make the crust
In a medium bowl, stir together the melted butter, sugar, vanilla, and salt. Add the flour and mix until just combined.
Press the batter evenly into the prepared baking pan by using a spatula or your hands and then smoothing out with a spatula. Bake at 300°F for 20 minutes. Meanwhile, make the filling and topping.
Make the filling
In a large bowl, gently mix together the thinly sliced apples, flour, granulated sugar, cinnamon, and nutmeg to evenly coat apple slices. Set aside.
Make the streusel topping
In a medium bowl, combine the oats, brown sugar, granulated sugar, cinnamon, and flour until all ingredients are welI blended. Using your hands or two forks, cut in the cold cubes of unsalted butter until the mixture resembles coarse crumbs. I find that using my hands and pressing the butter through my fingers as I mix it with the dry ingredients works best. Set aside.
Assemble and bake
Remove the crust from the oven. Increase heat to 350°F.
Layer the apple filling onto the crust, spreading it evenly throughout the pan. Move slices around until you get an even layer of filling.
Sprinkle streusel topping evenly over the filling layer.
Bake at 350°F for 45-55 minutes. The top will be a nice golden brown color, and you'll be able to hear the slight bubbling of the filling. And, your house will smell magical!
Remove from the oven and let rest for about 20-30 minutes to cool the bars to room temperature. Move to the refrigerator to chill for at least one hour before lifting out of the pan and slicing into bars.
Serving size: I slice these into small squares, which yields 24 bars. You may slice them larger or smaller, depending on how many people you are serving. Refrigeration: You can can refrigerate these apple pie bars for up to 2 days. Freezing: You can also freeze them for up to 1 month in an airtight, freezer safe container. When you're ready to serve, thaw the bars at room temperature.Serving ideas: I find these bars to be sweet enough on their own, but you can serve these delicious apple pie bars/apple crisp bars with vanilla ice cream, whipped cream, or drizzle caramel over the top.