Preheat oven to 200º. Line 2 cookie sheets with foil, lightly coat with nonstick cooking spray.
Pop 3 bags of microwave popcorn. Dump popped popcorn into a large bowl, try to remove any unpopped kernels. Add salt & peanuts if desired.
In a pan on the stove, bring brown sugar, butter, and corn syrup. Boil for 5 minutes, stirring continuously to avoid any burning. The color will start to lighten and the mixture with start getting almost a little frothy. Remove from heat and stir in baking soda.
Carefully pour the mixture over the popcorn & stir, making sure that everything is well-coated.
Spread the coated popcorn on the 2 foil-lined cookie sheets. Place in oven for 1 hour, stirring every 15 minutes.
Let cool and serve.
Store in airtight container or ziplock baggie.
Notes
The contents while stirring the caramel mixture will be VERY hot. So be extra careful, especially if kids are around. If you're using plain popcorn (instead of butter-flavored microwave popcorn), you may want to consider adding a little butter flavoring or some extra salt.