In a small bowl, mix together the mascarpone and creme fraiche with a spatula until well combined. Gently fold in the truffle pieces.
Halve the wheel of brie horizontally: Use a knife to score the halfway point between the top and bottom of the wheel. Holding a piece of kitchen twine firmly in both hands and starting at the score, pull it through the wheel to halve it. Remove the top and spread the mascarpone mixture on the bottom half of the wheel so that it is evenly distributed. Replace the top half and wrap tightly in plastic wrap. Refrigerate for at least two days and up to 1 week. The longer you let the cheese sit, the more intense the truffle flavor will become.
Remove from the fridge at least 10 minutes before you are ready to serve. Enjoy with fresh crusty bread – preferably a French baguette!