Adjust your oven rack to the second level mark and preheat your oven to 350ºF.
Generously spray a 13" x 9" baking pan with a flour-based baking spray. If you like, you can add a parchment paper sling to the pan for easy removal of the bars before cutting. Make sure to respray the top of the paper with the flour-based baking spray if going with this option.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set to the side.
Using a hand-held mixer or a stand mixer fitted with a paddle attachment, cream together the unsalted butter, granulated sugar, and light brown sugar on medium-high speed until well combined.
Add in the large eggs one at a time, until each one is fully mixed with the batter. Then add in the vanilla extract.
Reduce your mixer speed to low and gradually add in your whisked dry mixture until it is just combined. Do not over mix the cookie batter.
Using a spatula or spoon, add in the M&M's, chocolate chips, and broken pieces of pretzels. The cookie dough mixture should be thick and a little sticky.
Spoon the cookie dough into the 13" x 9" baking pan, then evenly spread it out until it reaches the edges. You can use your clean hands or a spatula sprayed lightly with some flour-based baking spray.
Place in the oven and bake at 350ºF for 35-40 minutes or until the top is a nice light golden brown.
Remove from the oven and allow to cool completely on a wire cooling rack before cutting into squares.