Preheat your oven to 350°F (175°C) and line the bottom of an 8x8-inch baking dish with parchment paper, using metal binder clips to secure the parchment paper to the sides. Lightly spray the sides and the bottom of the pan with cooking spray.
Line the Oreos on the bottom of the baking dish in an even layer, creating 4 rows of 4.
In a large mixing bowl, whisk together the melted and cooled butter and the packed light brown sugar until well combined.
Add the room temperature egg, egg yolk, and vanilla extract to the butter and sugar mixture. Mix until everything is well incorporated.
Using a rubber spatula, gently fold together the all-purpose flour, baking soda, baking powder, cornstarch, Christmas M&M candies, and semi-sweet chocolate chips until they are evenly distributed throughout the batter. Do not overmix; you want a thick blondie batter.
Evenly spread the blondie batter over the Oreo cookies in the baking dish.
Bake in the preheated oven for approximately 25-30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with a few moist crumbs. You want the top to be nice and golden.
Remove the baking dish from the oven and allow it to cool in the pan for about 15 minutes.
Use the overhanging parchment paper to lift the blondies out of the pan and place them on a wire rack to cool completely.
Once cooled, cut the blondies into squares and enjoy your delicious Christmas-themed Oreo M&M Blondies! Optionally, top with extra salt for added flavor.