Pop the corn and separate the fluffy bits from the hard bits and unpopped kernels. Discard the hard parts and set the fluffies aside with the nuts in a BIG bowl. As big as you can get.
Line a baking sheet with parchment paper and preheat the oven to 250ºF.
In a medium sauce pot, combine butter, sugars, salt, water, and any flavorings you like. Bring to boil over medium heat and boil 5 minutes until thickened. Stir frequently to prevent boiling over. It will bubble up quite a lot.
Remove from heat and stir in vanilla and soda.
Quickly pout the caramel over the popcorn and nuts and toss with a large spoon until fairly evenly coated. (You can serve it now if you’ll not be trying to store it for later munching.)
Spread out onto the baking sheet and bake for 30 minutes, giving it a stir every 10 minutes.