(makes enough icing for 10 personal-sized cakes)
¾ stick butter, soft
8 ounces cream cheese, soft
1 t orange zest
1 t vanilla
4 C powdered sugar, sifted

Make sure butter and cream cheese are soft before beginning. I prefer allowing it to come to room temperature rather than microwaving.
In the bowl of a stand mixer, combine butter, cream cheese and orange zest using the paddle attachment. Beat on high-speed until well combined. Add powdered sugar to butter mix and begin on slow speed. Increase speed as sugar becomes incorporated into the butter mix. Remove paddle and scrape down sides and bottom to insure a very smooth mix. Once sugar is incorporated, add vanilla and continue to beat until the icing has a very spreadable, smooth texture. The batter can hold in the refrigerator for several days. Allow icing to come to room temperature for smooth spreading.
Typo note: It looks like you say to use sour cream in the text, but the recipe and picture show cream cheese.
Oops. Thanks for pointing that out. Yes CREAM CHEESE. Will fix.
Brent