Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.

In a medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Set aside.

In a large bowl, combine the melted butter, light brown sugar, and granulated sugar. Using an electric mixer, beat on medium speed for about 1-2 minutes until well combined and smooth.

Add the eggs one at a time, beating well after each addition. Beat in the vanilla extract until just combined.

Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.

Reserve about 1/2 cup of the M&M's for the topping. Gently fold the remaining 1 cup of M&M's and the semi-sweet chocolate chips into the cookie dough with a spatula.

Scrape the dough into the prepared 9x13 inch pan. The dough will be thick. Use the spatula or your hands to press it evenly into the pan.

Sprinkle the reserved 1/2 cup of M&M's evenly over the top of the dough, gently pressing them in slightly. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs attached.

Let the cookie bars cool completely in the pan on a wire rack for at least 2-3 hours. This is crucial for clean cuts and a chewy texture. Do not attempt to cut them while warm.

Once completely cooled, use the parchment paper overhang to lift the entire slab out of the pan. Place on a cutting board and slice into 24 bars.