Preparation Steps

• Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper leaving an overhang for easy lifting, or lightly grease the pan.
• In a medium bowl whisk together the flour, baking soda, kosher salt, and cinnamon until evenly blended. Set aside.
• Using a stand mixer fitted with the paddle (or a large bowl and hand mixer), beat the butter with the brown and granulated sugars on medium speed until the mixture is pale and fluffy, about 3 minutes.
• Add eggs one at a time, beating until each is incorporated, then mix in the vanilla. Scrape down the bowl so everything is well combined.
• With the mixer on low, add the flour mixture in two additions, mixing just until no dry streaks remain. Do not overmix—stop as soon as the dough comes together.
• Fold in 1 cup of the M&Ms and all of the chocolate chips with a spatula so they are distributed evenly through the dough.
• Transfer the dough to the prepared pan and press it firmly and evenly into the corners with an offset spatula or the back of a spatula. Sprinkle the remaining 1/4 cup M&Ms on top and gently press them into the surface.
• For slightly taller, chewier bars, chill the pan in the refrigerator for 10 minutes before baking to reduce spreading. Then bake in the preheated oven for 20–26 minutes, or until the edges are golden and the center is just set (a toothpick inserted near center should come out with a few moist crumbs).
• Remove from oven and set the pan on a wire rack. Let the bars cool completely in the pan—at least 45 minutes—before lifting them out using the parchment overhang and slicing into squares.