In a medium bowl, use an electric mixer to beat the butter; beat just until creamy. Then add the cream cheese and beat until combined. IF YOU BEAT THE CREAM CHEESE TOO LONG, THE MIXTURE WILL BE TOO SOFT FOR PIPING.
Mix in the vanilla, then gradually add the powdered sugar- 1/2 cup at a time. Mix until the frosting ingredients are completely combined- do not over-mix. Check the consistency at this point. If it seems too soft, go ahead and add a little more sifted powdered sugar. If it seems too thick, mix in a teaspoon or so of milk. Use the frosting to frost your cake or cupcakes immediately- either by spreading or filling a piping bag, or store in a covered container in the refrigerator until ready to use (should be used to frost the same day).
I can't stress enough... if you over-mix, the frosting will be too soft for piping. Adding more powdered sugar will just make it softer.
Notes
Nutritional information is for the frosting only-- not the entire cupcake.
If you spread the frosting on the cupcakes instead of piping it on tall like the pictures show, you'll be able to frost 24 cupcakes.