Bring milk to a simmer in a saucepan; remove from heat and cool to room temperature.
Whisk buttermilk into milk; let sit at room temperature, 8 hours to overnight. Strain mixture through cheesecloth over a bowl in the refrigerator, 8 hours to overnight.
Be sure to let it cool to room temperature before whisking in the buttermilk or you will end up with a curdled, hot mess!
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