Preheat oven to 190°C.
Defrost puff pastry for approximately 15 to 20 minutes and unfold.
In a saucepan, melt the butter over medium heat. Saute the walnuts in the butter until golden brown, approximately 5 minutes. Add the cinnamon and stir until walnuts are coated well. Place the walnut mixture on top of the Brie and sprinkle the brown sugar over the mixture. Lay the puff pastry out on a flat surface. Place the brie in the center of the pastry. Gather up the edges of the pastry, pressing around the brie and gather at the top. Gently squeeze together the excess dough and tie together with a piece of kitchen twine. Brush the beaten egg over top and side of Brie. Place Brie on a cookie sheet and bake for 20 minutes until pastry is golden brown.
Serve with crackers.
To give a special look, cut extra pastry into heart or flower shapes and bake until golden.