In a medium bowl, combine pecans and sugar, using your fingertips to thoroughly mix; set aside. Line a baking sheet with a Silpat (French nonstick baking mat) or parchment paper; set aside.
Lightly dust work surface with flour. Place two sheets (one box) of puff pastry on the work surface one on top of the other. Using a rolling pin, adhere the two pieces of the puff pastry together. Continue rolling puff pastry until it is very thin, slightly less than 1/8 inch thick. Using a pastry wheel or a sharp knife, cut the puff pastry into a 13-inch round, reserving scraps for decorating.
Place the Brie in the center of the puff pastry. Top the Brie with pecan mixture, spreading to within 1/2 inch of the edge of the cheese. Be sure to remove any sugar or nuts from the puff pastry, as they may cause holes to form during baking. Fold puff pastry up and over Brie to enclose. The center of the Brie will not be completely covered with pastry. Transfer to prepared baking sheet.
Lightly dust work surface with flour. Repeat rolling and cutting process with remaining two sheets of puff pastry. Drape pastry over the Brie, tucking the edges under to enclose.
In a small bowl, combine egg yolk and cream to make an egg wash. Cut reserved scraps of puff pastry into decorative shapes. Using a pastry brush, gently brush the decorative shapes with the egg wash, and adhere to wrapped Brie. Brush the wrapped Brie evenly with the egg wash. Transfer to refrigerator and chill for at least 30 minutes and up to overnight.
Preheat oven to 400 degrees. Bake Brie until golden brown and puffed, about 30 minutes. Reduce oven temperature to 350 degrees and bake until dark golden brown, 25 to 35 minutes more.