Preheat oven to 250 degrees F and line a baking sheet with parchment paper. In a medium mixing bowl, stir together nut butter, syrup, vanilla, and spices. Microwave for 30 seconds, whisk, and microwave for another 30 seconds. Whisk again to make sure everything is evenly combined. Place popcorn and walnuts in a large mixing bowl. Pour liquid mixture over popcorn and walnuts and mix together (using your hands is best!) until the popcorn is evenly coated.
Scoop popcorn onto the prepared baking sheet. Bake for 60 minutes stirring every 15 minutes. Remove from oven and let cool completely before pouring into a bowl. Sprinkle chopped No Cow Bar onto popcorn and toss to combine!