In a large mixing bowl, combine Rice Chex, Corn Chex, pretzels, and mini marshmallows.
In a saucepan, melt butter over low heat. Stir in brown sugar and light corn syrup until dissolved. Remove from heat and add vanilla extract.
Pour the sweet glaze over the cereal mixture, tossing gently to coat evenly.
Spread the mixture onto a parchment-lined baking sheet.
Bake for 45 minutes, stirring every 15 minutes to ensure even caramelization.
Let the mix cool completely before adding M&M’s or other mix-ins.
Store in an airtight container for up to two weeks.
Notes
For a quicker method, microwave the mixture in a large microwave-safe bowl for 4-5 minutes, stirring every minute. Allow to cool before adding M&M’s or chocolate chips.