Preheat oven to 375°F (190°C) and line a large baking sheet with parchment paper.
Lightly flour the counter or cutting board and unfold both of the thawed puff pastry sheets.
Cut each puff pastry sheet into squares, aiming for about 4 inches per side.
Add ¼ cup of cranberry sauce to the center of each square and spread evenly.
Lay a few pieces of sliced brie on top of the cranberry sauce.
Sprinkle each square with chopped rosemary for added flavor.
Fold over the corners of the pastry squares to form triangles and seal the edges by pinching them together.
In a small bowl, add the beaten egg with a splash of water to create an egg wash. Brush this mixture over the top of the pastry triangles.
Bake in the preheated oven for 15-18 minutes, or until the pastries are golden brown.
Remove from the oven and allow to cool slightly before serving.