Preheat your oven to 350°F. Grease a 9×13 metal pan and set it aside.
In the bowl of a stand mixer, combine the melted butter, light brown sugar, and granulated sugar. Mix until well combined. Add vanilla and eggs, continuing to mix until fully incorporated.
While the mixer is on low, add in the flour, salt, cornstarch, and baking soda. Mix until just combined.
Use a spatula to fold in the Christmas M&M's and chocolate chips.
Scoop half of the batter into the prepared pan, using a spatula to ensure an even spread.
Thinly layer Oreos on top and cover with the remaining cookie dough.
Bake for 25 minutes or until the edges turn golden brown.
Remove from the oven and allow it to cool completely before indulging in this delightful treat!