Melt butter with sweet and condensed milk, and Corn syrup in a large pan (so it won't boil over).
Add sugar, stir. Cook on medium to low heat until it starts to bubble. Once it boils, cook for eight minutes, stirring constantly. Careful not to burn.
Remove from heat and add vanilla, stir.
Airpop a big bowl of popcorn (as much as wanted) and toss with warm caramel sauce until desired consistency is reached. While still warm sprinkle with flake salt*.
Leftover caramel can be refrigerated and makes an excellent caramel apple dip.
