1. Whip egg yolks at high speed in mixer until doubled in volume, about 10 minutes. They will be pale-yellow in color.
2. Meanwhile, place water in bottom of double boiler and heat until barely simmering.
3. In another saucepan, bring sugar, lemon juice and butter to boil.
4. When egg yolks are whipped, decrease mixer speed to medium. Gradually add hot suger-butter mixture, beating constantly.
5. Place mixing bowl just above water level over simmering water. (Do NOT let bowl touch water) Whisk mixture constantly and cook until mixture is thick custard, consistency of sourcream, about 10 minutes.