About 1 hour before making place a glass or metal mixing bowl in freezer.
Using a double boiler, combine sugar, egg whites and vanilla. Cook whisking constantly until mixture reaches 140* on a candy thermometer.
Pour egg mixture into chilled glass bowl and beat on high for 10 min. (I have a bosch and my bowl is plastic. It would have been better had I had a glass or metal bowl.)
Reduce mixer speed to medium low and add butter 1 TBS. at a time mixing well after each addition.
Mix in zest. Chill in refrigerater for about 30 minutes. Rebeat slightly then use.
TIP: You can make this ahead of time and keep it in the refrigerater. When you want to use it simply pull it out and bring to room temperature. Rebeat then use. I keep mine in a ziplock bag. It makes for less dishes and it is not so hard to transfer from.