ROUX: Heat a saute pan over medium heat. Add butter and allow it to melt. Sprinkle flour, black pepper, and garlic powder; whisk constantly for 75 seconds to cook out the raw flavor of the flour. Then add the 1 tsp mustard* and whisk to combine.
FILLING: Whisking with one hand, stream in ¾ cup of milk with the other hand. Allow the milk to start simmering, then add the chicken, and salt; stir to combine and let it heat through. If it looks significantly thick, add the remaining ¼ cup of milk while you stir. Allow the mixture to cook for 1-2 minutes.
LET STAND: Once the chicken mixture thickens, taste and adjust with more mustard, garlic powder, salt and pepper as needed. Then remove from stove. Filling will thicken further as it cools. Filling can be stored covered in the refrigerator for 24 hours before filling.
PREPARE THE SHELLS: Defrost pastry shells according to package directions. Cut the 5x5 squares diagonally to make two triangles. Fill one side of the triangle with 1 tablespoon of the chicken filling. Fold the other side over and crimp with a pastry crimper or press down with a fork (see image in post.) Repeat with remaining ingredients.
BAKE: Position a rack in the center of the oven and preheat the oven to 375ºF. Line a baking sheet with parchment paper. Place the puff pastry sheet pan and lightly brush with beaten egg. If the pastry is too soft, you’ll want to refrigerate it for at least 45-60 minutes before baking so they puff up. I usually bake from frozen. Bake for 25-30 minutes or until lightly golden brown. Do not open the oven for the first 15 mins.
FREEZE AND BAKE FROM FROZEN: Line the pastries on a baking sheet at least 1-inch apart and flash freeze for 2-3 hours. Then place in a ziptop bag and freeze. Bake from frozen, brushed with a beaten egg. Pastries will take longer when baking from frozen; about 27-35 minutes. DO NOT open the oven for the first 15 mins of baking.