1 recipe Gluten-Free Puff Pastry, chilled
FOR THE PASTRY CREAM
4 extra-large egg yolks
4 tablespoons cornstarch
2 cups milk (nondairy is fine, nonfat is not)
1/2 cup (100 g) sugar
1/4 teaspoon kosher salt
1 teaspoon pure vanilla extract
FOR THE APPLE FILLING
2 medium Granny Smith apples, peeled, cored and cut into 1/2-inch dice
2 tablespoons cornstarch
2 teaspoons freshly squeezed lemon juice
1/2 teaspoon ground cinnamon
1/4 teaspoon freshly grated nutmeg
2 teaspoons (8 g) sugar
1/8 teaspoon kosher salt
Egg wash (1 extra-large egg at room temperature, beaten with 1 tablespoon heavy cream)
Allow the puff pastry to sit out at room temperature until slightly softened, about 10 minutes. Divide the pastry dough in half. Place one half in between two sheets of unbleached parchment paper and roll it out into an 8-inch square a bit less than 1/4 inch thick, dusting with flour as you roll to keep the butter from melting or sticking. Slice into 4 pieces measuring 4-inches by 4-inches square. Repeat with the second half of dough. Return all of the pastry crust squares to the refrigerator to chill.
Make the pastry cream. In a medium-size, heat-safe bowl, whisk the egg yolks with the cornstarch until well-combined. Set the bowl aside. In a medium-size heavy-bottom saucepan, place the milk, sugar, salt and vanilla and whisk to combine. Bring to a simmer over medium-high heat. Pour about 1/4 cup of the simmering milk mixture slowly into the egg yolk mixture, and whisk until well-combined. Pour in more milk a few tablespoons at a time and continue to whisk. Once half the milk mixture has been added, add the rest of the milk more rapidly, continuing to whisk constantly. Pour the mixture back into the saucepan and cook over medium-low heat, whisking constantly, until thickened. Whisk the pastry cream until it is cool to the touch. Place plastic wrap directly on top of the surface of the pastry cream to prevent a skin from forming. Chill in the refrigerator until mostly firm, about 1 hour (or up to 3 days).
In a medium-sized bowl, place the diced apples. Add the cornstarch, lemon juice, cinnamon, nutmeg, sugar, and salt, and mix to combine. Set the bowl aside.
Remove the chilled pastry crust squares and the chilled pastry cream from the refrigerator. Preheat your oven 375°F. Line a rimmed baking sheet with unbleached parchment paper and set it aside.
Place 2 to 3 tablespoons of pastry cream in the center of each pastry square. Scatter about 10 pieces of apple on top of each dollop of pastry cream. Fold one corner of the pastry crust over the filling, and fold the opposite corner over the first. Brush each of the pastries generously with the egg wash. Place the pastries in the center of the preheated oven and bake for about 20 minutes or until evenly browned all over. Serve warm.