In a large bowl (or in the bowl of a stand mixer), mix the butter until it is creamy and smooth.
Mix in the powdered sugar, a little at a time, adding in the vanilla and 2 tablespoons of milk a little at a time too. If the frosting is too thick, add up to 2 tablespoons more milk until you achieve the thickness and texture you're looking for.
Notes
*For the whitest frosting you can get, use butter than is more white than yellow, and look for clear vanilla extract (in the baking aisle at craft stores, or at baking supply stores). You can certainly use regular vanilla extract if you are unable to locate clear.
This recipe makes enough to frost a two-layer cake or 24 cupcakes.