For the Chocolate Chip Cookies:

1. Preheat oven to 350°F (177°C). Line a baking sheet with parchment paper or a silpat mat.

2. Microwave the cold butter in 20-second intervals until halfway melted.

3. In a stand mixer, beat the semi-melted butter and both sugars on medium speed for 1-2 minutes.

4. Turn the mixer to medium-low, add eggs one at a time, and mix until just combined. Add vanilla and mix until incorporated.

5. In a separate bowl, whisk together flour, cornstarch, baking soda, and salt. Add to the mixer and mix until incorporated. Fold in chocolate chips.

6. Cover and chill dough in the refrigerator for 1-2 hours or overnight.

For the Brownies:

1. Prepare the brownie mix according to the package instructions or use a homemade brownie recipe (without the Oreo and fudge).

2. Allow brownies to cool to room temperature and chill in the refrigerator until firm.

3. Slice brownies into 1-inch chunks and set aside.

Assembling and Baking the Cookies:

1. Preheat the oven to 350°F (177°C).

2. Scoop ¼ cup of cookie dough, flatten it in your palm, and place a brownie chunk in the center. Wrap the dough around it and roll into a ball.

3. Place cookie dough balls on the prepared baking sheet, spacing them 2-3 inches apart.

4. Bake for 10-12 minutes, or until edges are set, and the center still looks undercooked. Rotate the sheet at the 6-minute mark for even baking.

5. Remove from the oven and immediately press more chocolate chips on top. Cool on the baking sheet for 15 minutes, then transfer to a wire rack.

Storage:
Store leftover cookies in an airtight container at room temperature.