How to Make It
Whisk the milk and vinegar together in a small bowl and let sit for 10 minutes.
Pour the heavy cream into a jar, then add the milk and vinegar mixture. Stir well to combine, then cover the jar (you can use a lid, or a cheesecloth kitchen towel secured with a rubber band).
Let the cream stand for at least 24 hours at room temperature, or up to 48 hours. This will give the lactic acid bacteria time to ferment, thicken, and sour the cream. The acidity of the mixture will prevent the dairy from spoilage. Chill prior to serving.