This is from chef Eric Ripert's new cookbook Avec Eric: A Culinary Journey with Eric Ripert as excerpted in the LA Weekly.
From: Avec Eric by Eric Ripert
Note: Makes about 2 cups. Ripert says the crème fraîche helps give his homemade version the flavor and texture of European butters.
1 quart heavy cream (preferably 40% fat)
1 cup crème fraîche
1 gallon ice water
1. Stir the cold cream and crème fraîche together in the bowl of an electric stand mixer. Using the mixer fitted with the whisk attachment, whip the cream on medium speed for 20 to 25 minutes, or until the fat separates from the liquid, making sure to cover the mixer and bowl with a towel or piece of plastic wrap.
2. Drain the butter through a fine-mesh sieve (reserving the buttermilk for later use) and rinse the butter in the ice water. Place the fresh butter on a clean, damp towel and knead on a cold surface to remove the rest of the moisture. Wrap the butter well and refrigerate. For best flavor, allow the butter to warm up to room temperature before serving.