Preheat oven to 225 degrees. Place popcorn in large roasting pan.
In a medium saucepan add butter, sugar, corn syrup and cream of tartar and stir frequently over medium heat until butter is melted and sugar is dissolved.
Bring mixture to a boil and continuously stir for about 3-5 minutes until until mixture is golden brown.
Remove from heat and stir in baking soda and ½ teaspoon coarse salt.