HEAT oven to 300°F. Spray rimmed baking sheet with no-stick cooking spray. PLACE popped popcorn in a very large bowl or Dutch oven. COMBINE peanut butter, corn syrup, butter and salt in a medium saucepan over medium heat, stirring constantly, until mixture begins to boil. Remove from heat, add vanilla and stir to combine. POUR warm peanut butter mixture over popcorn, stirring gently to combine. When all popcorn appears evenly coated with peanut butter mixture, spread evenly on prepared baking sheet. BAKE 20 to 25 minutes, stirring occasionally. Remove from oven and allow to cool. May be stored in airtight container for up to one week.