Preheat oven to 350°F.

Lightly dust a work surface with flour and roll out each pie crust to about 1/8-inch thickness. Repeat with the second crust.

Cut out 6 to 8 rectangles (around 2-inches wide by 3-inches tall) from each pie crust. Reroll the dough scraps into 1 or 2 more tarts if desired.

Transfer the rectangles to a baking sheet lined with parchment paper; leave 2 inches of space between them.

Spread a heaping tablespoon of Nutella on half of the rectangles.

Using a fork, lightly prick a couple of holes on the remaining rectangles.

Place the pricked rectangles on top of the Nutella-filled ones.

Crimp the edges of the tarts with a fork.

Whisk the egg yolk and water in a small bowl; brush each tart with the egg wash.

Sprinkle turbinado sugar on top of each tart.

Bake for 13 to 15 minutes, or until golden brown.

Let cool for a few minutes and serve.