In a mixing bowl, combine the egg whites, granulated sugar, cream of tartar, and vanilla extract. Beat until stiff peaks form to create a light meringue.
Roll out the thawed puff pastry on a lightly floured surface. Cut each sheet into rectangles, approximately 3x4 inches.
On half of the pastry rectangles, spread a layer of Nutella (about 1 tablespoon per rectangle). Add a dollop of marshmallow frosting and a square of chocolate on top.
Gently fold the meringue into the center of each filled rectangle.
Place the remaining pastry rectangles over the filled ones. Press the edges down firmly to seal and crimp with a fork.
Whisk together the egg and water. Brush the tops of the assembled Pop Tarts with the egg wash.
Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper and place the Pop Tarts on it. Bake for 15-20 minutes until puffed and golden.
While the Pop Tarts are baking, mix together the Nutella, heavy cream, and vanilla extract for drizzling. Once baked, let them cool slightly, drizzle the Nutella mixture on top, and sprinkle with graham cracker crumbs.
