I originally got the recipe here, but I've made minor changes.
Homemade Caramel Popcorn
8 cups popped popcorn (1 regular sized bag)
1/2 cup brown sugar
1/4 cup margarine
2 tablespoons light corn syrup
1/4 teaspoon salt (I omit this if I use already salted popcorn)
1/2 teaspoon vanilla extract 1/4 teaspoon baking soda
Pop the microwave popcorn per instructions on package. Once popped, remove the "duds." I do this by pouring the popcorn into a large bowl, shaking it, so all the duds fall to the bottom, then scooping the popcorn back into the bag in handfuls. Dump out the duds and return the popcorn to the large bowl.
In a large glass (or other heat proof) bowl, combine the brown sugar, margarine, corn syrup, salt and vanilla. Heat for 1 minute 30 seconds in the microwave, then take out and stir until well blended.
Return to the microwave, and cook for 30 seconds. Remove from microwave, and stir in the baking soda.
Pour syrup over the popcorn in the bowl. Stir to coat the popcorn as best as you can. Put the bowl of popcorn into the microwave for an additional 30 seconds. Remove, stir well, and return it back to the microwave for another 30 seconds. Remove, stir well.
Dump the popcorn out onto waxed paper, and let cool until coating is set. Enjoy! Store in an airtight container, if there's any left!