Preheat the oven to 200 degrees.
Pop the corn, pick out the unpopped kernels and set aside in a large bowl.
Add the cashews to the bowl, if using.
On the stovetop in a medium-sized saucepan, combine the brown sugar, butter, salt, cream of tartar, and corn syrup.
Bring to a boil, stirring constantly, and boil for 4-5 minutes.
Remove saucepan from heat and stir in the baking soda and vanilla. The mixture will foam up.
Add the mixture to the popcorn and cashews.
Stir to coat the popcorn and cashews thoroughly.
Spread onto rimmed baking sheets or large broiler pans and place in the oven for 1 hour.
Stir the cashew caramel corn every 15 minutes.
When the hour has passed, spread the caramel corn on wax paper or foil to cool.
When the cashew caramel corn is cool, break it apart.
You can serve it or store it in plastic zipped storage bags.