Straight from the Super Secret Family Recipe Book:
2 C Brown Sugar
1 Cube Butter (1/2 cup)
2/3 C Condensed Milk (I always just use the whole can. I never measure it)
1/2 C Karo Syrup
Pinch of Salt
Mix those ingredients in a microwave safe bowl. Cook in microwave for 5 minutes. Stir. Then add 1 TSP vanilla extract, cook for another 5 minutes in the microwave.
Pour over a large bowl of popcorn. I normally do 3 batches of air popped popcorn. If you want it extra gooey, then do 2 batches.