Preheat your oven to 350°F (175°C) to ensure even baking.
Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, cream the softened unsalted butter, brown sugar, and granulated white sugar until light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the bowl and mix until well combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring gently with a spatula until just combined. Do not overmix.
Fold in the broken pretzels, peanut butter chips, and M&M's, reserving a few M&M's for topping.
Spread the batter evenly in the prepared pan and press the reserved M&M's and small pretzel twists on top.
Bake for 25-30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
Remove from oven and let cool in the pan for 10 minutes. Lift out using the parchment paper and cool completely on a cooling rack before slicing.