Monster Cookie Bars

Preheat the oven to 340° F (171℃). (Note: We are baking at a slightly lower temperature to better control the rise of the bars). Prepare a 9×13 baking pan with nonstick cooking spray and parchment paper. Cut the parchment paper so that it extends up past the sides of the pan.
In a medium mixing bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with an electric hand mixer, mix together the melted unsalted butter, light brown sugar, and granulated sugar until well combined and smooth. Add in the eggs and vanilla extract and mix until well combined. Then, add in the peanut butter and mix until fully incorporated and smooth.
Add the dry ingredients and mix on low speed until the cookie dough forms. Fold in the quick oats, 1 cup of semisweet chocolate chips and 1 ¼ cups of M&M candies until they are well dispersed throughout the cookie dough.
Use an offset spatula to spread the cookie dough into an even layer in the prepared 9×13 baking pan. Press the additional ½ cup of chocolate chips and ½ cup of M&Ms into the top of the cookie dough.
Bake the monster cookie bars for about 35-37 minutes until the edges of the bars are golden-brown and the center is very slightly underbaked. The bars will finish baking as they cool in the pan. Cool the bars completely to room temperature before removing them from the pan and cutting into squares.