Homemade Buttermilk

Here's how: In a mason jar, combine 1/2 cup cultured buttermilk (the supermarket stuff is okay here; it will act as a starter), 2 cups whole milk, and 1/4 teaspoon kosher salt. Cover and let stand in a warm place until the buttermilk thickens and coats the sides of the jar when tipped, 24-36 hours. Keep chilled for up to 2 weeks.