Preheat the oven to 250 degrees F (120 degrees C). Grease two shallow baking pans. You may use roasting pans, jelly roll pans, or disposable roasting pans.
Spread popped corn and nuts if using evenly in baking pans.
Combine the brown sugar, corn syrup, margarine, and salt in a saucepan. Bring to a boil over medium heat, stirring enough to blend. Once the mixture begins to boil, boil for 5 minutes while stirring constantly.
Remove from the heat, and stir in the baking soda and vanilla. The mixture will be light and foamy. Immediately pour over the popcorn in the pans, and stir to coat. Don't worry too much at this point about getting all of the corn coated.
Bake for 1 hour. Stir halfway through if you'd like.
Line your countertop with wax paper. Dump the popcorn out onto the waxed paper and separate the pieces. Allow to cool on the counter, while you heat chocolate.
Melt chocolate according to package directions. Pour over caramel corn and mix together to coat the caramel corn completely. Let it rest on the counter until the chocolate is hardened.
Serve immediately or package. You can store in an airtight container, but I think this gets a little chewy a few days later, so I like to store in a paper bag. But trust me, unless you make a quadruple batch this will not last more than a day!