Microwave Caramel Corn

This is my friend Bobbi's recipe:

1 stick margarine (or butter)
1 cup brown sugar
1/4 cup light corn syrup
1/2 tsp salt
1 tsp vanilla
16 cups unsalted popped corn

Combine in an 8 cup microwaveable bowl: margarine, sugar, salt, and corn syrup. Stir together and microwave on high for 3 minutes, stirring after each minute. Add vanilla and stir. Place popcorn into a brown paper bag (like a big grocery store bag) - non recycled bags hold up best, or use 2 paper bags, one inside the other. Pour mixture onto popcorn in the bag. Fold top of bag down and microwave on high 1 minute, then shake the bag, then micro for 1 minute, and shake, then 30 seconds, and shake, and another 30 seconds, and shake. Pour the popcorn out into a bowl or large cookie sheet to cool. Break apart when cool and serve.