Sour cream

Lactic acid is added to fresh sweet cream to make sour cream. Sour cream is a thick, tangy product used for dips, baking, salad dressings or a general condiment. Despite its name, sour cream is not "spoiled" cream.

You can purchase this common ingredient in most grocery stores.  Or see this recipe to make your own sour cream at home.

As a substitute, whole milk yogurt is interchangeable with sour cream but cakes and muffins will have a slightly drier texture.
OR Use equal quantities of drained yogurt (drain through cheese cloth)
OR Combine 7/8 cup buttermilk or yogurt plus three tablespoons butter or margarine

8 ounces sour cream = 1 cup