Heat oven to 350 degrees F. Prepare a baking sheet by lining it with parchment paper or misting with cooking spray.

Sprinkle a cutting board or other flat surface lightly with flour. Then spread out the puff pastry sheet so that it is flat. Use a rolling pin to roll out the puff pastry until it is smooth and even. Use a pizza cutter or knife to then cut the puff pastry along the three folding lines into thirds. Then cut each long rectangle into two long triangles, making six triangles total.

Place a dallop (about 1 tablespoon) of Nutella on the wide end of the triangle, and use a spoon to spread it out just a bit over the triangle. (Try to leave at least a 1/2-inch border all the way around the edges of the triangle, though, so the Nutella does not seep out during baking.) Then, beginning with the wide end of the triangle, roll the dough up into a croissant shape. Place on a baking sheet, and turn in the edges of the dough slightly to make a “U” croissant shape.

Repeat with the remaining five triangles. Then repeat with the second sheet of puff pastry and remaining Nutella.

In a separate bowl, whisk together the egg and one tablespoon of water until combined. Then liberally brush the egg mixture onto the top of each croissant. (You can also use the egg wash to help seal the ends of the croissant, if they are not holding together.)

Bake for 20-25 minutes, until the croissants are golden and flaky. Serve immediately or store in a sealed container for up to 3 days.