Heat a large skillet over medium heat and melt the butter.

Add the apples, brown sugar, and cinnamon. Stir, cover, and cook until the apples are tender, about 3-5 minutes. Be sure to stir occasionally to prevent burning.

Add the cornstarch slurry to the apple mixture and stir to combine. Cook for another 1-2 minutes uncovered, or until slightly thickened.

Stir in the salt. Taste the apple mixture and add more sugar, cinnamon, or salt to taste, if desired. Remove the pan from the heat and set aside.

Preheat your air fryer to 350°F.

Cut the pie crust into 5 to 6-inch diameter circles. When you run out of room to cut more circles, form the excess dough into a ball and roll it out with a rolling pin to about 1/8-inch thick. Repeat until you run out of dough.

Place 1 to 2 tablespoons of the apple filling into the center of each circle, being careful not to overfill. Brush a thin layer of the whisked egg along the edges surrounding the filling. Fold the crust in half, line up the edges, and seal with a fork, pressing about 1/4-inch from the edge. Repeat until you run out of pie crust or apple filling.

Brush the top of each pie with a thin layer of egg wash, then cut 2-3 slits into the top with a small knife (or poke with a fork), and sprinkle with sanding sugar.

Spray your air fryer basket with a nonstick cooking spray and arrange the pies in a single layer. Air fry for 10 minutes, or until the crusts become golden brown. Depending on how large your air fryer is, you may need to do this in batches.

Remove the pies from the air fryer and allow them to cool for 5 minutes on a wire rack.

Serve as is or with caramel sauce, ice cream, or whipped topping, if desired.