Mix softened butter in a small bowl with a fork until smooth
Add the powdered sugar a little at a time, mixing until it is fully incorporated.
Stir in the vanilla extract and milk until the frosting is smooth and spreadable.
Divide between two completely cooled cupcakes and spread with the back of a spoon.
Notes
Powdered sugar: Powdered sugar is also called icing sugar or confectioners' sugar.Storage: This frosting is best used when you make it. If you need to prepare it ahead, cover it tightly and refrigerate it for up to four days. Let it soften at room temperature, then stir until smooth before frosting the cupcakes.