5-6 quarts of air-popped popcorn (or 2 3.5 ounce bags of plain microwave popcorn)
Preheat the oven to 225 degrees Fahrenheit.
Have your popped popcorn at the ready in a very large metal or glass bowl. We used two medium metal bowls. Also, if you are using microwave popcorn (which we did...who needs to go and buy a fancy air popper) sift through to get all the unpopped kernals out before mixing. Add the nuts and the chopped candy bars.
In a large heavy-bottomed saucepan melt the butter, brown sugar, corn syrup and salt over medium heat. Once melted, allow the mixture to come to a boil without stirring until it reaches 250 degrees on a candy thermometer (about the firm ball stage). Remove from the heat and quickly stir in the baking soda and vanilla extract. The mixture will foam a bit and turn a creamier color. Pour the caramel mixture over the popcorn and nut bowl and mix quickly to coat the popcorn in the delicious caramel. The heat will melt some of the candy bar pieces but leave some intact. Yum!
Place the entire mixture in a large nonstick roasting pan (like the one you use for making chicken and turkey) and bake in the low-heat oven for one hour, stirring occasionally with a rubber spatula.
Allow to cool and try to only eat some.