For the Marshmallow Coating:

1. In a medium saucepan, melt the butter over medium heat. Add the brown sugar and corn syrup and cook, stirring constantly, until melted and combined, about 2 minutes. Add the marshmallows and stir until completely melted. Remove the pan from the heat and stir in the vanilla, salt, and cinnamon.

For the Mix:

1. In a large bowl, toss together the popcorn, pretzels, cereal, and nuts. Spread the mixture out in an even layer on two large baking sheets lined with parchment paper.
2. Carefully pour the marshmallow coating over the mixture, coating it as evenly as possible. Gently toss with a spatula until everything is evenly coated.
3. Once the mixture has mostly cooled, add the mini marshmallows, M&Ms, candy corn, and sprinkles. You still want it a little bit warm so these things adhere to the popcorn.
4. Store the snack mix in an airtight container for up to a week. It should stay soft and slightly gooey.