Special equipment: small 3 to 4 cup saucepan Candy thermometer Bi-therm or Insta-read thermometer
In small saucepan, combine sugar with 1/3 cup water, and cream of tartar. Stir to wet sugar. Cover and bring to a simmer over medium heat. Simmer, covered, without stirring, for about 1 minute to dissolve sugar. Uncover and wash down the sides of the saucepan with a wet pastry brush. Cook until syrup registers 242 degrees (the upper end of the soft-ball stage) on a candy thermometer. Or cook until you can blow a bubble (resembling a soap bubble) by dipping a slotted spoon into the syrup, holding it up, and blowing gently but steadily through it.
Meanwhile, in a heatproof, stovetop-safe bowl (a stainless steel electric mixer bowl is ideal, otherwise a stainless steel bowl suitable for using with a hand held electric mixer) beat eggs until pale and thick; set aside. Just before syrup reaches 242 degrees, begin beating eggs again on medium speed. When the syrup is ready, remove it from the heat and pour it in a thin steady stream over the eggs, beating constantly. To avoid splashing hot syrup on yourself or all around the sides of the bowl, pour it next to the beaters. Stop beating and place mixture on the stove over medium heat. Cook, stirring constantly with a whisk (to avoid scrambling the eggs) until mixture registers at least 160 degrees on insta-read thermometer, about 3-4 minutes. Remove from heat and continue beating with an electric mixer until mixture cools to room temperature.
Divide butter into 1-tablespoon sized lumps. Add butter, a few pieces at a time, continuing to beat until mixture is smooth and spreadable.