Preheat oven to 250°F (120°C). Line a large baking sheet with parchment paper or a silicone mat.

In a large mixing bowl, combine Rice Chex, Corn Chex, pretzels, and mixed nuts.

In a medium saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup, and salt. Bring to a gentle boil for 2–3 minutes, stirring constantly.

Remove from heat and stir in vanilla extract and cinnamon if using.

Pour hot butter-sugar mixture over cereal mixture. Fold gently until evenly coated.

Spread coated mixture onto the prepared baking sheet. Bake 1 hour, stirring every 15 minutes.

Cool completely on the baking sheet. Mix will harden as it cools.

Melt chocolate and butterscotch chips in microwave, stirring every 30 seconds. Drizzle over cooled mix.

Once chocolate sets, toss in M&M’s and mini marshmallows if desired.

Store in an airtight container at room temperature for up to 2 weeks.