1. Preheat the Oven:
- Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone baking mat.
2. Cream the Butter and Sugars:
- In a large mixing bowl, beat the cold cubed butter, brown sugar, and granulated sugar together with a hand or stand mixer until just combined (about 1-2 minutes).
3. Add the Eggs:
- Mix in the eggs one at a time, beating just until combined.
4. Combine Dry Ingredients:
- In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
- Gradually add the dry ingredients to the wet mixture, mixing just until incorporated.
5. Fold in the Mix-Ins:
- Stir in the chocolate chips and walnuts until evenly distributed. The dough will be thick and chunky.
6. Shape the Cookies:
- Divide the dough into large balls, about 6 oz (170 g) each (or roughly the size of a baseball). Do not flatten; the tall shape helps achieve a gooey center.
7. Bake:
- Place the dough balls on the prepared baking sheet, spacing them a few inches apart. Bake for 8-10 minutes, or until the edges are golden brown but the centers still look slightly underbaked.
8. Cool and Serve:
- Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. Enjoy warm for the ultimate gooey texture.