In a sauce pan, add 1 cup of water and cornstarch. Whisk these together before you turn on the pan! Add in juices, essential oil (if you are using it) sugar, egg and zest. Turn heat on medium low and whisk while cooking, until mixture thickens and begins to bubble (About 10 minutes). Remove from heat and whisk in cold butter and vanilla. Pour into bowl and let cool before use. Sauce will thicken up and be spreadable after it cools.