Preheat your oven to 425 degrees F and place a rack in the center.
Carefully slice the rind off the top of the brie round (it's OK if there is some left on). In a small bowl, make the egg wash by beating the egg with 1 tablespoon water with a fork or whisk until it's very smooth. Set aside.
Lay the puff pastry sheet out on a piece of parchment paper. Use a rolling pin to roll out the seams and make it as square as possible (but don't roll it out much bigger than it already is). Use a sharp knife or pizza cutter to cut the corners off so it's a circle (reserve the scraps). Place the cranberry sauce (1/3 cup) in the center of the circle and spread out so it's about 4.5-5 inches in diameter. Place the 8 oz. Brie round cut side down on top of the cranberry sauce. Fold the sides of the circle up to meet in the center, going around in one direction, until the brie is completely covered. Press the seam together as best you can (don't worry if it doesn't stick together perfectly and if it looks a little messy!) and flip over so the smooth side is up in the center of the parchment paper. Transfer the whole piece of parchment paper onto a rimmed baking sheet, lifting from each side so the brie is in the center of the baking sheet.
Use a pastry brush to brush the top and sides of the puff pastry wrapped brie with the egg wash. Optional: Use a sharp knife to cut leaves and roll berries out of the scraps of puff pastry. If you like, you can score lines for the veins of the leaves. Place them on top of the wrapped brie and press down gently to adhere them to the egg on top. Brush the tops of the leaves and berries with egg wash.
Bake at 425 degrees F in the center of your oven for 20-30 minutes, or until golden brown. If baking unevenly, rotate the baking sheet halfway through cooking.
Allow to cool for at least 30 minutes before serving. Serve with water crackers or crostini, with a small cheese knife for slicing wedges.
No rolling pin? Use a drinking glass to smooth the seams.
To use a larger round of brie, roll the puff pastry out to be bigger to accommodate for the bigger side. Also, spread the cranberry sauce out to a larger diameter, and you might need to use more.
Cranberry sauce substitution: use apricot jam, orange marmalade, or another jam of choice in place of cranberry sauce.