Beat all ingredients in a medium bowl with mixer on medium-low speed until blended. Increase speed to high; beat until smooth and fluffy.

Lemon Variation: Prepare vanilla frosting with the following changes: Omit milk and vanilla extract; add 1 tsp grated lemon peel and 1/4 cup lemon juice. (To give frosting a more lemony color, beat in 3 drop yellow food color.)