~from Savoring Time in the Kitchen
Please note that you will have to start the crème fraîche at least 30 hours before you need to use it. It needs 24 hours to thicken to the right consistency plus 6 hours of chilling time.
8 ounces/1 cup heavy (whipping) cream (Do not use ultra pasteurized)
4 ounces/1/2 cup regular sour cream, at room temperature, or, 1 tablespoon Buttermilk
1 tablespoon confectioners sugar (optional). (Use if you prefer a slightly sweeter version for dessert use, omit for a more savory version)
Place all ingredients into a glass jar with lid and shake thoroughly to blend. Remove the lid and cover the jar with a small cloth or piece of paper toweling secured with a rubber band or twine.
Leave on the kitchen counter at room temperature for 24 hours, stirring once or twice.
After the mixture has thickened sufficiently (it will thicken faster in a warm room), stir, cover tightly and refrigerate for another 6 hours before using.
Adapted from What's Cooking America