Prepare the Baking Dish: Line a 9x13-inch baking dish with parchment paper, leaving some overhang on the sides for easy removal later. Lightly grease the parchment paper with butter or non-stick spray.
Melt the Marshmallows: In a large saucepan or pot, melt the butter over medium heat. Add the mini marshmallows and stir continuously until completely melted and smooth. Remove from heat and stir in the vanilla extract and a pinch of salt.
Combine the Ingredients: Quickly add the Rice Krispies cereal to the melted marshmallow mixture, stirring until evenly coated. Add the broken pretzel pieces and half of the M&Ms, and mix until well distributed.
Press into the Baking Dish: Transfer the mixture to the prepared baking dish. Using a spatula or your hands (lightly greased with butter), press the mixture firmly and evenly into the dish.
Add the Toppings: While the mixture is still warm, sprinkle the chocolate chips evenly over the top. Allow them to sit for a minute or two until they begin to melt, then spread the melted chocolate evenly with a spatula. Sprinkle the remaining M&Ms over the melted chocolate, pressing them gently into the surface.
Cool and Set: Allow the treats to cool at room temperature until the chocolate has set. You can also place the baking dish in the refrigerator to speed up the cooling process.
Cut and Serve: Once fully set, use the parchment paper overhang to lift the treats out of the baking dish. Place on a cutting board and cut into squares or bars.