HEAT oven to 350 degrees F. Line rimmed baking sheet with foil.
COMBINE butter, peanut butter, vanilla and cinnamon in a microwave-safe bowl. Microwave on HIGH (100% power) 35 to 45 seconds, stirring until well blended.
COMBINE cereal, mixed nuts and pretzels in large bowl. Pour butter mixture over cereal mixture. Toss well to coat. Spread mixture evenly on foiled baking sheet.
BAKE 10 to 12 minutes, stirring occasionally. Cool. Sprinkle lightly with cayenne pepper, if desired. Toss. Store in resealable food storage bag.