How to Make It
Preheat oven to 300°F. Combine popcorn, cereal squares, pretzel squares, and walnut pieces in a large bowl.
Place sugar, butter, and syrup in a small saucepan over medium. Cook, stirring occasionally, until mixture comes to a boil, 3 to 4 minutes. Reduce heat to medium-low, and cook, stirring often, until sugar is completely dissolved, 1 to 2 minutes. Remove from heat, and add vanilla and baking soda, stirring constantly. Pour syrup mixture over popcorn mixture, stirring gently until well combined and evenly coated.
Spread popcorn mixture in a single layer on a parchment paper-lined rimmed baking sheet. Bake in preheated oven until golden and caramelized, 25 to 30 minutes, stirring after 15 minutes. Remove from oven, and let cool completely, about 15 minutes. Transfer to a serving bowl, and stir in candies.