Preheat the oven to 200 degrees and pour the Chex into one large bowl and the popcorn into another large bowl.
Make the caramel sauce by adding the butter, brown sugar, corn syrup, and salt to a small saucepan. Cook over medium heat, stirring constantly with a wooden spoon. Melt the butter then allow the mixture to simmer for 5 minutes, stirring the whole time.
Remove the pan from heat and add the baking soda. Stir to mix the baking soda into the other ingredients, then immediately pour the mixture over the Chex and popcorn.
Carefully stir the cereal and popcorn until it’s fully coated in the caramel sauce. Give it a little toss, toss, toss and a shake, shake, shake to make sure your caramel sauce gets in all those nooks and crannies!
Spread the mixture evenly over 2 parchment paper lined baking sheets and top with Christmas sprinkles, if desired. Place the pans in the oven to bake for 1 hour. Set your timer to toss your cereal mixture on 20 minute intervals to allow for even browning.
When the cook time is up, remove the pans from the oven and allow to cool completely.
Top with M&Ms and mini marshmallows, then transfer to a serving bowl or airtight container.