Place the popped corn in a very large bowl, or two large bowls, leaving some room to stir. Be sure to remove all kernels.
In a heavy saucepan over medium heat, stir together butter, corn syrup and brown sugar.
Bring mixture to a boil and boil for 3-4 minutes, stirring constantly.
Remove from heat and stir in can of sweetened condensed milk.
Return to heat and boil for 3 more minutes, stirring constantly.
Test for soft ball stage by placing small drop of the hot caramel into a small bowl of cold water.
Roll the caramel from the water into a ball with your hands and it will give you the consistency of the finished product. You want the caramel to be a soft ball that can hold its shape.
Remove from heat again, let it sit for 2 minutes then pour over prepared popcorn and stir until well coated.
Eat as is or make into balls using buttered fingers and placing the balls on a sheet of waxed paper until cool.
Store in an airtight container.