Bring butter, brown sugar and corn syrup to a boil in a heavy pot over medium to medium high heat, stirring constantly. Slowly stir in sweetened condensed milk. Continue stirring until mixture returns to a full boil. As soon as caramel returns to a boil, start timer for 5 minutes and continue stirring until the timer goes off. Remove from heat. Pour the caramel mixture over the popped corn and stir until well combined. Allow to cool before serving.