Combine sugar and water in a medium saucepan; place over low heat, stirring until sugar is dissolved. Increase heat; continue stirring until mixture comes to a boil. Stir in cream of tartar.
Wash off crystals that have formed on sides of pan, using a brush dipped in cold water. Cover and boil vigorously for 3 minutes. Uncover; cook rapidly, without stirring, until mixture reaches soft ball stage (240°). Immediately remove from heat; pour mixture out onto a damp counter or marble slab, leaving any crystallized sugar in pan. Let syrup cool.
Using a spatula, pull sides into middle repeatedly until mixture begins to thicken and turn white.
Knead fondant with buttered hands until creamy enough to form a firm ball. Sprinkle with kirsch or Grand Marnier; knead lightly. Store in a tightly covered contained for at least 4 days before using.