Make caramel according to the recipe but only thicken on the shorter side (about 5 minutes). Once thickened remove from heat and add in the baking soda with the vanilla and salt. You will see it bubbling but this is what helps it be more spreadable.
Add popcorn to a large bowl, with the caramel and nuts (if using). Mix around to coat well.
Spread popcorn mixture out on a parchment lined cookie sheet.
Bake for an hour, mixing around every 15 minutes.
Once done, remove from the oven and allow to cool. Once cool you can break it apart.
Serve! Stores crisp in an air tight container!
Notes
The baking soda helps the caramel be smooth and easy to spread over the popcorn.