Preheat oven to 275 degrees F. Coat a large, rimmed baking sheet with cooking spray and set aside. Pop 12 cups of popcorn in a large, heavy-duty saucepan. Set aside.
In a medium saucepan, melt the butter over medium heat. Add the corn syrup and brown sugar and cook until mixture is smooth and is amber in color, 4 to 5 minutes.
Remove from heat and immediately add the marshmallow creme and vanilla. Stir until smooth. Pour the mixture over the popcorn and carefully stir with a large spoon until the caramel is evenly distributed and popcorn is well-coated. Fold in the Baby Ruth pieces.
Pour the popcorn onto the prepared baking sheet and bake in the preheated oven for 20 minutes, occasionally reaching in the oven to stir and flip. Remove from oven and let cool. Break off in pieces. Store in an air-tight container.