In a medium saucepan, combine the brown sugar, butter, corn syrup and salt and bring to a boil. Add the baking soda and cream of tartar and boil over moderate heat, stirring occasionally, until the syrup is deep amber, about 4 minutes. Pour the hot syrup over the peanuts and popcorn and stir with a wooden spoon until evenly coated. Spread the mixture on the baking sheet and bake until slightly dry, about 1 hour. The caramel corn will harden as it cools; let it cool completely.