In a large deep saucepan, combine the brown sugar, corn syrup, butter, salt, and cream of tartar. Inset a candy thermometer. Bring to a boil, and cook the mixture over medium heat for about 5 minutes, stirring occasionally, until the thermometer registers 260 degrees F. (hard ball stage). Remove from heat, and quickly stir in the baking soda. The mixture will foam up quite a bit.