Combine sugar, water, salt, and cream of tartar in a heavy duty 2 quart saucepan; cook over medium heat, stirring constantly, until the sugar dissolves.
Cover and cook 2 to 3 minutes to wash down sugar crystals from the sides of the pan.
Remove cover, and cook without stirring, to a soft crack stage.(270 degrees or just a bit more) Watch carefully for it to turn light amber. If it turns dark amber, it is burnt. Add the butter and whisk it in quickly.
Remove from heat; add cream and whisk it inches.
Serve warm over ice cream, fresh fruit, cake, or apple crisp.