1. Combine brown sugar, water, and butter in saucepan; bring to a boil. Cover and boil (over medium-low to low) for 2-3 minutes. Begin popping corn.s
2. Remove sugar mixture from heat, and add vanilla. Let caramel sauce cool just slightly, stirring constantly with spoon. Pour over popped popcorn in a large bowl, and toss/fold with wooden spoon until well combined and glazed.
3. Let cool (it helps to put it in fridge), and it gets nice and crunchy. Or if you simply can't wait, dive in immediately, but it will be wet and messy.
You can also substitute peanut butter (or other nut butter) for margarine (increase the amount), and add some cinnamon. Experiment.