Cut the puff pastry into 18 squares by cutting each sheet into 9 equal pieces.
Place the puff pastry squared on a baking sheet lined with parchment or a baking mat.
Spread 1/2 teaspoon of dijon mustard down the center of the square from corner to corner on the diagonal. Repeat with the remaining squares.
Sprinkle approximately 1 tablespoon shredded cheese on top of the dijon and top with a slice of ham.
Dab a bit of the egg yolk on one of the corners of each pastry square not covered with topping and fold the opposite corner over the filling. Fold the remaining corner with the egg yolk over the pastry and press lightly to seal.
Lightly brush the tops of each croissant with the egg yolk and sprinkle with parmesan and garlic powder.
Transfer the baking sheet to the oven and bake for 20-25 minutes or until puffed up and golden brown.
Allow the croissants to cool for 2-3 minutes and serve hot or room temperature.