Add the cold cubed butter, brown sugar, and white sugar to a mixing bowl. Use a hand mixer to mix just until the butter and sugars are evenly combined. The mixture should look creamy and slightly textured, but it does not need to be whipped until fluffy.

Do not mix for too long. Overmixing at this stage can add too much air to the dough and may cause the cookies to spread more during baking.

Add the medium egg and vanilla extract to the butter mixture. Mix again with the hand mixer just until everything is fully combined.

Scrape down the sides of the bowl if necessary so the egg is evenly incorporated. The mixture may look slightly uneven at first, but it will come together as you continue mixing.

Add the all-purpose flour, cornstarch, baking soda, baking powder, and salt to the bowl. Switch from the hand mixer to a silicone spatula and gently fold the ingredients into the wet mixture.

Continue folding until the dough begins to come together. Do not mix until the dough is completely smooth, as a small amount of visible flour is fine before adding the chocolate and Oreos.

Break the Oreos into a combination of crumbs and small pieces. Add the Oreo pieces and white chocolate chips to the bowl while there are still a few visible streaks of flour in the dough.

Fold gently with the spatula until the chocolate and cookies are evenly distributed and the dough is just combined. Be careful not to crush all of the Oreos into fine crumbs. Larger pieces create a better texture and make the cookies look more appealing after baking.

While the dough chills, preheat your oven to 180°C. Line one or two baking trays with parchment paper.

Make sure the oven is fully preheated before baking. A properly heated oven helps the cookies spread evenly and develop lightly golden edges while keeping the centers soft.

For the large cookies, place only four dough balls on each baking tray. Leave plenty of space between them because the cookies will spread as they bake.

Bake the large cookies at 180°C for approximately 11 to 12 minutes, or until the edges are lightly golden while the centers still look soft and slightly underbaked.

For the smaller cookies, bake at 180°C for approximately 8 to 9 minutes. Keep a close eye on them because smaller cookies can become overbaked quickly.