In a large bowl, use an electric mixer to beat the butter and cream cheese at medium speed until creamy. Gradually add the powdered sugar, beating at low speed until blended. Increase the speed to medium, and slowly add in the milk and vanilla, beating until smooth. Beat in drops of color if you wish to tint the frosting.
Notes
This recipe will frost 24 cupcakes at 2 tablespoons of frosting per cupcake. If you're more generous with your frosting, you may not be able to frost 24 with a single recipe.