Pop popcorn kernels and place in large bowl.

Place butter, brown sugar, and Karo syrup in medium saucepan over medium heat. Stir occasionally until butter is melted and mixture is smooth. Bring to a rolling boil. Once boiling, reduce heat to low and simmer for 5-7 minutes, stirring constantly. The mixture should turn a caramel color.

Remove from heat and add the salt and sweetened condensed milk. Stir until smooth. Pour over the popcorn and stir to evenly coat the popcorn.