3/4 c. sugar
1/4 c. water
1/8 tsp. cream of tartar
3 egg yolks
1 c. unsalted butter (or wash the salt out of salted butter)

Boil water, sugar and tartar to 236 degrees. Beat yolks until light. Add to hot syrup and beat until cold and thick. Whip butter until fluffy and add to the syrup. Mix and spread on cake.