1. Spread popped popcorn on large cookie sheet (figure A).2. Preheat oven to 250 degrees Fahrenheit.
3. In a saucepan, melt butter and stir in brown sugar, corn syrup and salt. Stir constantly until mixture comes to a boil. Continue to boil without stirring for about 5 minutes.
4. Remove from heat and add baking soda and vanilla. Keep stirring until foamy. Add nuts, stirring until heated through and no longer clumped.
5. Pour caramel mixture over the popcorn and stir to coat (figure B). Bake for 30 minutes, stirring two or three times to evenly distribute the coating (figure C).