|heavy cream, preferably not ultra-pasteurized|
|buttermilk, preferably full fat|
In a jar with a tight-fitting lid, stir together the cream and buttermilk. Place the jar in a warm spot, such as the top of the refrigerator or near the range, and allow the mixture to sit undisturbed until thickened but still pourable, 12 to 14 hours, or as long as 36 hours for cream that is ultra-pasteurized. The crème fraîche will continue to thicken on chilling and will keep for about 3 weeks refrigerated. When ready to whip, add the sugar and whisk lightly until soft peaks form when the whisk is raised. You can also use a handheld mixer on low speed.
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