Preheat your oven to 400°F (200°C).
Roll out the puff pastry on a lightly floured surface.
Cut the pastry into 3-inch squares.
In a bowl, mix the mozzarella, pepperoni, Parmesan, and Italian seasoning.
Scoop a tablespoon of the filling onto the center of each pastry square.
Fold the pastry over to form a triangle and pinch the seams to seal.
Place the sealed puffs on a baking sheet lined with parchment paper.
Brush the tops of the puffs with the beaten egg.
Bake for 15-20 minutes or until golden brown.
Let cool slightly before serving.