Preheat oven to 250 degrees. Line a baking sheet in parchment paper. (I know, it sounds fussy, it makes clean up so easy.)
Combine pretzels and cereal in a large bowl. Melt the butter in the microwave. Let sit for one minute to cool.
Combine cinnamon and sugar in a small bowl.
Pour the melted butter over the cereal a little at a time and stir. Add some more and stir. Stir until all the cereal and pretzels have a fine layer of butter on them. Sprinkle 1/3 of the sugar mixture over the cereal and stir. Then add the next third of sugar and stir some more.
Pour mix on the baking sheet and spread evenly. Sprinkle with remaining cinnamon sugar.
Bake for 18 minutes and cool completely. Sprinkle with M&Ms and serve.