" Try making your own crème fraîche—it couldn’t be easier. Just combine 2 tablespoons of crème fraîche or cultured buttermilk with 2 cups of heavy cream. Pour into a clean glass jar, cover loosely or leave uncovered, and leave out at warm room temperature for 2 days, or until it thickens. "

― Samin Nosrat , Salt, Fat, Acid, Heat: Mastering the Elements of Good Cooking