Prepare the baking sheet by lightly spraying with cooking spray.
Mix pretzel, popcorn and walnuts and chop roughly (use a food processor or put them in a kitchen towel or a fabric bag and lightly smash them with the meat beater; I'm rudimentary like that! lol). Then transfer to a large glass bowl.
In a medium saucepan bring half and half to a boil, reduce heat to low and add marshmallows. Stir in until completely melted using a spatula.
Meanwhile make the caramel sauce. Pour sugar in a medium large saucepan and add water. Bring to a boil then turn the heat on low. Keep on heat until the sugar gets an amber color (not brown, that means it's burnt and it will taste super funky). Keep the marshmallow sauce on the stove until the caramel sauce is ready.
Once caramel sauce is done, slowly pour marshmallow sauce over, stirring with a whisk. Be careful, the combination will bubble at first. Mix until well combined.
Immediately pour over the popcorn pretzel walnut mixture, in small batches and mix well with a non stick spoon. Continue pouring and mixing until all ingredients are well coated.
Transfer to the prepared baking sheet, firmly press down (I used the side of a mug), then sprinkle M&M's and gently press the M&M's into the popcorn mixture using your hands.
Immediately cut into bars. If you pressed down the mixture well, you'll be able to easily remove the bars from the baking sheets.
That's it! Enjoy your super delicious sweet and salty popcorn bars!