Pop two LARGE bowls of popcorn (if it ends up being too much, just keep some out.)

In a saucepan, melt the butter.

Add the brown sugar and syrup. Stir constantly over medium heat until the sauce reaches the "soft ball" stage on your candy thermometer.

Remove from heat and add the sweetened condensed milk and vanilla. Stir.

Pour over the bowls of popcorn.