Step 1

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.

Step 2

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).

Step 3

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.