1. Mix the sugar and cream of tartar. Add the water and bring to a boil, stirring until the sugar dissolves. Continue boiling to 246° (a drop of the mixture forms a firm ball in cold water). Set aside to cool to lukewarm.
2. Beat the egg yolks in an electric mixer or with a rotary hand beater until very thick and fluffy. Add the syrup slowly while beating. Beat until cool.
3. Add the butter, a tablespoon at a time, beating well after each addition. Beat until very smooth and creamy. Chill until firm enough to spread. After frosting, chill the cake until ready to serve.