Preheat oven to 400 degrees.
Work quickly with thawed pastry or it gets sticky and difficult. A box has two sheets, let them thaw for about 30 minutes, then roll them on a floured board with a floured rolling pin until about 10 inch square. Cut each square into four squares and lay on a cookie sheet covered with parchment paper.
Layer a slice of brie and 3 slices of Granny Smith or other tart apple in the middle of each square.
Overfilling is the enemy, don’t overdo it.
Top each with a sprinkle of brown sugar, a pinch of cinnamon and a pinch of butter (1T cut into 8 pieces.). Brush the border of each square with a wash of whisked egg. Fold two opposite corners together and press to hold. Brush the entire top of pastry with egg wash.