Preheat oven to 375F (190C), and line a baking sheet with parchment paper.
Place the 6 puff pastry squares equally apart on the baking sheet.
Lightly score each puff pastry square diagonally across the middle, making two triangles, don’t cut through, just mark it.
On one half of the square, spread a heaped teaspoon of Nutella, leaving space around the border. On the other half of the square, sprinkle 6 mini marshmallows, keeping them in the middle.
Fold the square, making a filled puff pastry triangle. Use a fork to very tightly crimp and seal the edges shut.
Whisk together the egg and water making an egg wash, and brush this generously over the s’mores triangles.
Bake for 15-20 minutes or until top is golden brown. If you want these extra gooey, once they are out of the oven, open them carefully and add a few more mini marshmallows, you’ll be able to easily reseal them if you do this while hot.
Microwave a couple extra tablespoon of Nutella for 10-15 seconds or until just melted, then drizzle on the top of the turnovers. Sprinke the graham cracker crumbs on top. If desired, dust with a little powdered sugar.