Prepare popcorn on the stovetop or in an air popper according to package instructions. Spread popped popcorn on a cookie sheet.
Preheat the oven to 200°F degrees. Spread the popcorn out onto 2 large baking sheets. Remove any unpopped kernels. Set aside.
In a medium pot, melt butter over medium-high heat. Add sugar, corn syrup, cream of tartar and salt. Stirring constantly, bring mixture to a boil and continue to boil for 5 minutes.
Remove saucepan from heat. Stir in baking soda. The mixture will become thick and bubbly very quickly.
Pour caramel over popcorn, coating generously. Place baking sheets in the oven and bake for 1 hour, stirring halfway through to make sure popcorn doesn't burn.
Remove from oven and allow to cool completely. Mix in the candy corn and other toppings if desired.