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Instructions
Preheat the oven to 200 degrees F (95 degrees C).
Combine popcorn and cashews in a large roasting pan.
Combine brown sugar, butter, corn syrup, salt, and cream of tartar in a large saucepan; bring to a boil, stirring constantly, until a candy thermometer inserted into middle of syrup, not touching bottom of saucepan, 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. Form into balls while still warm, if desired.