Preheat oven to 350 degrees F.

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

Cut out 6 to 8 rectangles (around 2-inches wide by 3-inches tall) from each pie crust. Reroll scraps if needed.

Transfer the rectangles to a baking sheet lined with parchment paper, leaving 2 inches of space.

Spread a heaping tablespoon of Nutella on half of each rectangle.

Lightly prick the remaining rectangles with a fork for venting.

Place the vented rectangles on top of the Nutella-coated ones.

Crimp the edges 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 before serving.