Preheat oven to 375°F and line 2 large baking sheets with parchment paper and set aside.
In a large bowl or stand mixer (highly recommend a stand mixer for this recipe) fitted with a paddle attachement, cream together the butter and sugar on medium speed for 2 minutes, scraping down the sides of the bowl halfway through.
1½ cups unsalted butter, 1 cup granulated sugar, 1½ cups light brown sugar
Add the eggs to the butter mixture one at a time, mixing after each one just until the yolk is streaky throughout the dough. Add the vanilla extract and mix just until incorporated.
2 large eggs, 1 teaspoon vanilla extract
In a medium mixing bowl, whisk together the flour, cornstarch, salt, and baking soda. With the mixing beating at low (stir) speed, slowly add the flour mixture. If you have a flour shield for your mixer, I recommend using this as this recipe creates a lot of dough. Increase to medium speed and mix just until combined then remove from the mixer and use a rubber spatula to mix up any loose flour from the bottom of the bowl into the dough.
4 cups bread flour, 1 tablespoon cornstarch, 1½ teaspoons salt, 1 teaspoon baking soda
Add the M&Ms and chocolate chips and mix at low speed just until combined, be careful not to overmix or to mix at too high a speed or the M&Ms may crack. You can finish mixing with a rubber spatula or with your hands until the dough is fully combined and the chips are evenly incorporated.
¾ cup chocolate chips, 1¼ cup M&Ms
Use a large cookie scoop to portion out the dough on the prepared baking sheets, leaving 4 inches between each (about 6 to 8 cookies will fit on a standard cookie sheet). Gently press extra chocolate chips and M&Ms into the tops of the cookies.
Bake the cookies one cookie sheet at a time for 8 to 10 minutes, or until the edges are just turning light brown, the centers should still be soft and look underdone. Remove from the oven and allow them to cool for 5 minutes on the pan before transferring them to a wire rack to cool completely. Then transfer the cooled cookies to parchment paper.