Combine first 6 ingredients in top of a double boiler. Cook over simmering water, stirring constantly, until butter melts. Gradually stir about one-fourth of hot mixture into egg yolks; add to remaining hot mixture, stirring constantly. Cook over simmering water, stirring constantly, until mixture thickens and coats back of a spoon (about 15 minutes). Remove from heat; cool. Cover and chill.
I made this to dress up some white cupcakes since I had 4 egg yolks to use from having made the cake. Very tasty. Not too strong but sweet & tart at the same time. I didn't have the lemon zest since I used bottled lemon juice but added a 1/2 tsp of lemon extract.
If you're not a fan of standard frosting on a cake, this will hit the spot.
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