Preheat oven to 250 degrees F. Place popcorn in a very large bowl.
In a medium saucepan over medium heat, melt butter. Whisk in brown sugar and corn syrup. Bring to a gentle boil, stirring constantly. Then gently boil without stirring for 2 minutes. Remove from heat and stir in baking soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.
Place in two large shallow baking dishes and sprinkle with sugar and/or sprinkles. Then bake in preheated oven, stirring every 15 minutes, for about 45 minutes to 1 hour. Remove from oven and let cool completely before breaking into pieces.