1 cup heavy or whipping cream, room temperature
1 tablespoon buttermilk or 1/2 cup sour cream, room temperature
In a jar with a lid, place whipping cream and buttermilk (or sour cream); cover securely and shake 15 seconds. Set aside at room temperature for 24 hours or until very thick. Stir once or twice during that time. NOTE: Cream will thicken faster if the room is warm.
Stir thickened creme fraiche well. Refrigerate at least 6 hours before serving. Cover tightly and store in refrigerator for up to 2 weeks.