Cream butter and shortening together with an electric mixer.
Add vanilla and 1 cup of sifted sugar at a time. Beat well on medium speed scraping bottom and sides often. Frosting will appear dry. Add milk and beat at high speed along with a few drops of food coloring (optional). Add more milk for thinner consistency.
Frosting can be refrigerated in an airtight container for 1 week. Rewhip before using.
Notes: Clear vanilla extract may be used, if desired. For a large cake double the recipe.