Cut defrosted puff pastry into 18 squares by cutting each sheet into thirds, and cutting each third of a sheet into a third to make squares.Place each square on a greased or parchment lined baking sheet.
Spread ¼ teaspoon of dijon diagonally across each square.
Place about 1 Tablespoon of chopped ham on top of the dijon, followed by 1 Tablespoon of shredded cheese onto each square.
Spread a small amount of egg yolk on one corner of each pastry square. Make sure it's NOT one of the corners with the ham/cheese/mustard. Fold the opposite corner over the filling, then fold the egged corner on top of it and press to seal. Repeat for all pieces of puff pastry.
Brush the top of each pastry with egg yolk.
Bake at 350° F for 20-25 minutes, or until the pastries are golden brown.