In a small saucepan over low heat, heat sour cream just enough to warm through; remove from heat and allow to cool.
In a small jar, place the warm sour cream and the buttermilk; mix thoroughly with a whisk.
Cover the jar loosely and let the mixture stand at room temperature for 12 to 24 hours or until the mixture is thick and tart.
Refrigerate for longer storage.
Crema Agria Recipe - Mexican Sour Cream: https://whatscookingamerica.net/sauces_condiments/cremaagria.htm