Start by lining a 9x13 inch baking dish with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over medium heat, combine the 1/2 cup of unsalted butter and the 10-ounce bag of miniature marshmallows. Stir continuously until the mixture is smooth and fully melted.
Once the butter and marshmallows are melted together, remove the saucepan from heat and stir in the 1 cup of creamy peanut butter until fully combined.
In a large mixing bowl, combine the broken pretzel rods, Corn Chex cereal, and 1 cup of M&Ms. Pour the melted marshmallow mixture over the dry ingredients and stir until everything is evenly coated.
Pour the mixture into your prepared baking dish. Use your spatula to press the mixture down firmly, ensuring it’s evenly spread out.
If desired, sprinkle the additional 1/4 cup of M&Ms and 1/2 cup of miniature marshmallows on top of the pressed mixture for a fun and colorful finish.
Allow the bars to cool at room temperature for at least 30 minutes. For quicker setting, you can place them in the refrigerator for about 15-20 minutes.
Once set, use the overhanging parchment paper to lift the bars out of the baking dish. Slice them into squares or rectangles, and indulge in your delicious Chocolate Peanut Butter Chex Mix Bars!
