- Preheat oven to 250ºF and line a large baking sheet with parchment paper. In a medium saucepan, melt butter over medium heat, about 3 minutes. Stir in brown sugar. Bring to a low boil, stirring constantly for 5 minutes, until smooth and thoroughly mixed.
- Remove from heat and stir in soy sauce, vanilla extract, and baking soda.
- Slowly pour caramel over popcorn in a large bowl and stir gently to coat. Spread popcorn on baking sheets in a single layer.
- Bake for 30 minutes, stirring every 15 minutes. Remove from oven and cool completely at room temperature. Store in a dry spot in an airtight container.