Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
In a large saute pan, melt the butter over medium heat. Add the apples and saute until slightly softened, about 5 minutes. Add the brown sugar, cinnamon and salt and toss. Continue cooking the apples, stirring occasionally, until the apples break down easily when pressed with a fork, an additional 10 minutes. Remove from the heat and mash them with a fork while they are still warm, it should look like a chunky applesauce. Allow the apples to cool to room temperature. Once cooled, fold 1/4 cup of the Almond Pastry Cream into the cooled apples and set aside.
On a floured work surface, lay out the puff pastry sheets. Using a large (4 1/2-inch) circular biscuit cutter (or cookie cutter) cut 4 large circles per sheet, 8 all together.
On the bottom half of each of the rounds, spoon 2 heaping tablespoons of the apple mixture. Brush the edges of each circle with some egg wash. Fold the pastry dough over the filling, seal the edges with your fingers and then crimp them with the tines of a fork. Repeat with remaining pastry dough and filling. When all the pies are assembled, vent each one with 2 small slits, using a paring knife, on the top of each pie. Brush each one with the remaining egg wash and sprinkle with turbinado sugar. Transfer the empanadas to the prepared baking sheet.
Bake the pies until they have puffed and are golden brown, about 20 minutes. When the pies come out of the oven, spoon more of the Almond Pastry Cream over them and serve.
Almond Pastry Cream:
In a large, heavy-bottomed saucepan, combine the whole milk, sugar, almond paste and salt. Whisk over medium heat and bring to a simmer.
In a small mixing bowl, combine the egg yolks and cornstarch. Whisk until combined and the mixture is creamy and thick. Remove the milk mixture from the heat and slowly add 1/4 cup of the milk liquid to the eggs, whisking constantly. Add the now-tempered egg mixture back into the saucepan with the rest of the milk and return the milk to a simmer for 3 minutes, whisking constantly, until thickened.
Remove the saucepan from the heat and stir in the butter, 1 tablespoon at a time, whisking until it is fully incorporated. If desired, strain the pastry cream through a sieve. Pour the mixture into a bowl to cool. Place plastic wrap directly on the surface to prevent a skin from forming. Allow the cream to cool completely in the refrigerator until cold and set, at least 2 hours.