Ingredients

　　Buttermilk

　　Combine the cream with the buttermilk (1 tablespoon for each cup of cream) in a saucepan over medium heat. Heat just until the chill is off—to about 50°F. Pour into a glass jar, cover lightly with a piece of waxed paper, and let sit in a warm place (65°F to 70°F) for 12 to 20 hours, until the crème fraîche has thickened. Replace the waxed paper with plastic wrap or a tight-fitting lid and refrigerate for at least 6 hours before using. It keeps for up to 2 weeks.

　　How to Repair Food, Third Edition