1. Preheat your oven to 350°F. Line 2 large baking sheets with parchment paper and set aside.
2. In a large bowl, combine flour, cornstarch, baking soda, salt, and cubed butter.
3. Using a fork, pastry cutter, or just your hands, cut the butter into the dry ingredients until it is very fine and you no longer have any pieces of butter larger the about the size of a pea. (The smaller you can get the butter, the better!) Set aside.
4. In a medium sized bowl, combined granulated and brown sugar, egg, milk, and vanilla. Whisk to combine.
5. Pour the wet ingredients over the butter mixture. Mix together until you have a thick cookie dough. If the cookie dough is too soft, mix in a bit more flour (1 tablespoon at a time) until firm enough to scoop.
6. Add your chocolate chips and Oreos to the plain cookie dough, and fold together until combined.
7. Use a large cookie scoop to portion out your cookie dough, and drop the scoops onto your parchment lined baking sheet (leaving at least 2 inches distance between each cookie dough ball). Gently press each ball of dough down slightly to ensure even spreading.
8. Bake in the center of your preheated 350°F oven for 12 to 18 minutes, or until lightly golden brown along the edges.
9. Cool the cookies for 5 minutes on the baking sheet, then transfer onto a wire rack and set aside to cool before enjoying!
