Sour cream was traditionally made by allowing cream to sit at room temperature causing it to sour. Nowadays the cream is pasteurized and the bacteria needed to sour it are killed. Therefore the milk spoils before souring. This is a recipe for people who have access to unpasteurized milk. If you do not, you can still make Creme Fraiche. It is very similar but more milk in flavor.
1 cup heavy cream
(or 1/2 cup heavy cream and 1/2 cup whole milk for less fat)
1/4 sour cream/ buttermilk or vinegar
In a mason jar, combine the ingredients and shake up. Let sit, covered, at room temperature for 24 hours. By then it will be very thick.
Refrigerate before using. It will last a week in the fridge.