With an electric mixer at low speed, mix the powdered sugar and butter together. Once combined, add vanilla and milk. Increase speed of mixer to medium and beat for 1 to 2 minutes or until smooth and of spreading consistency.
To frost a cake, first brush away any loose crumbs from cooled cake. If frosting layers, place the bottom layer rounded side down on a plate. Spread about 1/3 cup of frosting to with 1/2-inch of cake's edge.
Place the top layer rounded side up. Coat the side of the cake first with a thin layer of frosting to seal in the crumbs, then frost the side making a rim about 1/4-inch high around the top.
Finally, spread remaining frosting on top, just to the frosting-made rim.
Servings: One 13x9x2-inch cake or fills and frosts two 8 or 9-inch cake layers