Heat oven to 250°F. Line two rimmed baking sheets with parchment. Place popcorn in a large bowl.
Melt butter in a medium saucepan over medium heat.
Stir in brown sugar, corn syrup, molasses, and salt. Bring to a gentle boil, stirring until smooth.
Simmer 4–5 minutes without stirring until glossy and slightly thickened.
Remove from heat. Quickly stir in vanilla and baking soda; the caramel will foam.
Immediately drizzle caramel over popcorn. Toss with a spatula until evenly coated.
Spread popcorn on prepared sheets. Bake 45 minutes, stirring every 15 minutes.
Cool completely on the pans. Break into clusters and store airtight.