In a large mixing bowl, combine 1/2 cup of softened unsalted butter, 1/4 cup of sour cream, and 1 teaspoon of pure vanilla extract. Using an electric mixer on medium speed, beat these ingredients together until they are well combined.
Gradually add in the powdered sugar, starting with 3 cups. Continue to mix on low. Once incorporated, increase the mixer speed to medium and beat until smooth.
If the frosting is too thick, add a bit more sour cream, one tablespoon at a time, until you reach a spreadable consistency. If it's too thin, add more powdered sugar a quarter cup at a time until it thickens enough.