In a large mixing bowl or stand mixer fitted with the paddle attachment, beat the butter on medium speed for 2–3 minutes until it’s light and fluffy.
Add the sour cream and vanilla extract. Beat on medium speed just until combined and creamy.
Reduce the mixer speed to low and gradually add the sifted powdered sugar, 1/2 cup at a time. Scrape down the sides of the bowl as needed.
Once the sugar is incorporated, increase the speed to medium and beat for 1–2 minutes until the frosting is smooth and fluffy. If the frosting seems too thick, blend in 1 tablespoon of sour cream (or milk) at a time until spreadable. If it’s too thin, add more powdered sugar, 1/4 cup at a time, until you reach the right consistency.
Use immediately to frost cakes, cupcakes, or cookies.