Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Roll out the puff pastry sheets on a lightly floured surface to about 1/8-inch thickness. Cut each sheet into 4 equal rectangles, yielding 8 total pieces.

Place 4 of the rectangles on the prepared baking sheet. Spoon about 2 tablespoons of hazelnut spread onto the center of each, leaving a 1/2-inch border around the edges.

Brush the edges of the pastry with the beaten egg, then top each with another rectangle of puff pastry. Press the edges firmly with a fork to seal.

Brush the tops with more egg wash and sprinkle with granulated sugar. Use a sharp knife to make 3 small slits on top of each tart for steam to escape.

Bake for 18–20 minutes, or until golden brown and puffed. Let cool slightly before serving.