Preheat oven to 400 degrees F.
Line a baking sheet with parchment paper - set aside.
Sprinkle a clean working surface with flour and rollout puff pastry until it’s about ½ an inch in thickness.
Use a paring knife to cut the puff pastry into squares, about 2 inches x 2 inches in size. From 397 grams of puff pastry, you can get approximately 30 squares.
Gently place each puff pastry square on the lined baking sheet. Refrigerate while you prepare the mushroom and cheese filling.
Melt butter on a small non-stick skillet over medium heat. Add cut mushrooms and cook for about 2 minutes, until sweating. Remove from heat.
In a medium-sized bowl, use an electric mixer to slowly beat cream cheese and egg. Once well combined, add parmesan cheese and fold.
Remove puff pastry from the refrigerator. Use an offset spatula to scoop about ½ tbsp of cream cheese mixture on the center of each puff pastry. Top off with mushroom mixture.
Bake for 17-20 minutes. Allow to cool for a few minutes (and to allow the puff pastry to deflate a little). Serve and enjoy!