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.