Combine light brown sugar, light corn syrup, butter, cream of tartar, and salt in a large saucepan.

Stir the mixture over medium-high heat until it begins to boil.

Continue stirring constantly and boil rapidly until the mixture reaches the hard-ball stage (260°F), approximately 5 minutes.

Remove the saucepan from the heat.

Quickly but thoroughly stir in baking soda.

Pour the caramel mixture over a large bowl of popcorn.

Gently stir until all the popcorn kernels are evenly coated.