Preheat the oven to 350 degrees and line a cookie sheet with parchment paper. Set aside.
Mix the flour, cornstarch, salt, and baking soda in a small bowl. Set aside.
Cream the butter, brown sugar, and white sugar with a hand mixer or in a stand mixer on medium speed until creamy. About 90 seconds. Then, add the egg and vanilla and mix again until just combined. About 20 seconds.
Slowly add the dry ingredients to the wet and mix until just combined.
Fold in the Oreos, chocolate chips, and M&Ms with a rubber spatula.
Using a 2-inch cookie scoop, scoop pout balls of dough and place them on the prepared baking sheet, leaving room for the cookies to spread.
At this point, add more crushed Oreos and M&Ms to the top if you like.
Bake for 10 to 11 minutes until the edges just begin to turn light brown.
Cool the cookies on the baking sheet for 5 minutes. Then, transfer the cookies to a wire rack to cool completely.
Notes
Storage
Store at room temperature for up to a week in an airtight container.
Place cookies in an airtight, freezer-safe container in single layers separated by parchment paper and freeze for up to 3 months.
Bring to room temperature before serving.
Tips
Soften your butter to room temperature. It shouldn't be warm or the cookies will be flat.
Your cookies will be underdone when you remove them from the oven. Carryover cooking time finishes them off.
If possible, measure your flour by weight.
If you don't have a food scale, loosely spoon your flour into your measuring cup. Never scoop flour out directly with the cup.