Combine sugar and water in a 3-quart saucepan; cook over low heat, stirring until sugar dissolves. Continue to cook over medium heat, stirring until mixture comes to a boil. Stir in cream of tartar.
Cover and cook 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and continue to cook until mixture reaches soft ball stage (234°).
Remove from heat; pour mixture out onto a marble slab that has been sprinkled with cold water. (Do not scrape pan.) Cool to lukewarm (110°).
Using a wide metal spatula, scrape sides into center mass until opaque and creamy.
Knead fondant, with buttered hands, until creamy enough to form a firm ball. Place in a small bowl. Cover tightly, and refrigerate at least 24 hours before using.