Pop popcorn into a large bowl and throw away any unpopped kernels.
Combine corn syrup and peanut butter in a small saucepan. Over medium heat, melt and whisk gently until smooth. Mixture will begin to bubble up slightly- allow it to gently boil for about 3-4 minutes, then remove from heat. Add in salt and vanilla.
Pour over popcorn and gently stir to coat peanut butter mixture. Add in candy and peanuts, if desired. I like to add in half the chopped SNICKERS bars, give it several stirs to mix, then add the other half.