Prepare 1 gallon (4 qts) of popped popcorn.
Preheat oven to 200°
Prepare two cookie sheets with silicone mats, parchment paper, or by buttering them.
Divide the popcorn and spread it across both pans in even layers.
Heat the butter, sugar, corn syrup, and salt in a heavy bottom pot over medium heat, stirring occasionally until bubbly around the edges.
Once bubbling, cook for 5 minutes, stirring occasionally.
Remove from heat and add baking soda. Stir vigorously to incorporate. It will get frothy when it is ready.
Divide the candy evenly over the two prepared trays of popcorn, tossing to coat as much as possible without breaking the pieces too much. It's ok if it's sticking together here.
Bake the trays in the oven for an hour total, stirring and breaking apart the big chunks every 15 minutes. If it's still tacky at the end of one hour, bake another 15 minutes.
Cool and serve. Store in an airtight container.