Place the popcorn and pretzels on a large baking sheet that has been lined with a Silpat baking mat. Set aside.

In a medium saucepan, melt the butter over medium heat. Add the brown sugar and corn syrup and cook until melted and combined. Add the marshmallows and stir until completely melted. Remove from heat and stir in the vanilla and salt.

Evenly pour the marshmallow caramel mixture over the popcorn and pretzels. Gently stir until the popcorn and pretzels are well coated. It will be sticky! Stir in the M&M's. Taste and season with additional salt, if desired. Store in an airtight container for up to one week. The popcorn will stay sticky and soft!

Note-if you need the recipe to be gluten-free, make sure you use gluten-free pretzels. You can also use peanut M&M's or stir in peanuts!