Blend all frosting ingredients, except for xanthan gum, until very smooth. Scrape sides often. Slowly sprinkle in xanthan gum. Blend bit longer.
Refrigerate for at least 6 to 8 hours until completely stiff. Briefly mix with hand mixer just until soft. Frost immediately and store in cool place. (It will melt faster than regular frosting.) Leftover refrigerated frosting will need to be whipped before using.