Place the popcorn in a large bowl. Shake and toss the bowl to make the unpopped kernels go to the bottom; discard.
In a sauce pan over medium-high heat, cook the brown sugar, corn syrup, and butter, stirring constantly. Heat to 270 to 290 degrees F (132 to 143 degrees C), or until a small amount of syrup dropped into cold water forms hard but pliable threads. Remove from the heat, and carefully pour in the can of condensed milk; stir until smooth.
Pour 1/4 of the caramel at a time over the popcorn, stirring until all of the popcorn is covered. Cool at least 10 minutes before serving.