Mix popcorn and cashews together in a large roasting pan.
Combine brown sugar, corn syrup, butter, salt, and cream of tartar in a large saucepan. Bring to a boil, stirring constantly, until a candy thermometer inserted into the middle of the syrup, not touching the bottom, reads 260 degrees F (127 degrees C), 6 to 8 minutes.
Remove syrup from heat; stir in baking soda. Pour over popcorn and cashews; stir to coat.
Bake in the preheated oven, stirring every 20 minutes, until sugar mixture has baked and caramelized onto popcorn and cashews, about 1 hour. Spread cashew caramel corn onto a parchment paper-lined baking sheet to cool. If desired, form into balls while still warm.