Pop popcorn, and place in a large roasting pan. Mix in any desired additions (cereal, pretzels, nuts, etc).
In a medium saucepan, add butter, brown sugar, salt and corn syrup.
Cook over medium-high heat (not high), stiring occasionally until the mixture begins to boil.
Boil 5 minutes, or until the mixture is a deep amber color, stiring occasionally. Set a timer, and do not leave the kitchen. The mixture can burn quickly. Keep the stove temperature just high enough to keep the mixture boiling, and no more (this may be medium, or medium-high). If you prefer to use a thermometer, the mixture should be about 234*F, or soft ball stage.
After boiling 5 minutes, remove pan from heat. Add vanilla and stir (the mixture will hiss). Add baking soda, and stir well (the mixture will foam and expand).
Pour caramel over popped corn (or any other ingredients you choose), and stir to coat.
Bake caramel corn in a pre-heated 250*F oven for about 25-30 minutes. Every 10 minutes, remove from oven, stir well and return to oven.
Spread the baked caramel corn onto a couple of cool baking sheets, or a waxed paper lined counter, to cool. Cool about 5 minutes. Eat immediately, or store in a sealed container 3-4 days.
If you want to include M&M's, candy corn, or other candies, add them after the caramel corn has baked, and cooled.