1. Spray a large shallow roasting pan with cooking spray. Add popcorn and place in preheated 250°F oven while preparing caramel.
2. Mix brown sugar, corn syrup, butter and salt in a heavy 2-quart saucepan. Stirring constantly, bring to a boil over medium heat.
3. Boil 5 minutes WITHOUT STIRRING. Remove from heat. Stir in baking soda and vanilla.
4. Pour syrup mixture over warm popcorn, stirring to coat evenly.
5. Bake for 45 minutes, stirring occasionally. Remove from oven and spread on foil that has been sprayed with cooking spray. Cool; break apart. Store in tightly covered container.
6. To serve, set out small bowls of your favorite candies, nuts or fruits and let your guests individualize their own Halloween treat.