Step 1: In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
Step 2: In a large bowl using an electric mixer, cream together the cold unsalted butter (cut into small cubes), granulated sugar, and light brown sugar on medium speed for about 2-3 minutes, until light and fluffy. Do not overmix.
Step 3: Beat in the large eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough.
Step 5: Fold in the semi-sweet chocolate chips and chopped walnuts or pecans (if using) by hand until evenly distributed throughout the dough.
Step 6: Divide the dough into 8 equally large portions (each should be about 6 ounces or 170g). Roll each portion into a tall, round ball. Do not flatten them; they should be high and chunky.
Step 7: Place the dough balls on a baking sheet lined with parchment paper. Cover loosely with plastic wrap and chill in the refrigerator for at least 30 minutes, or up to overnight. This step is crucial for thick cookies.
Step 8: Preheat your oven to 400°F (200°C). Arrange the chilled cookie dough balls on a baking sheet lined with parchment paper, leaving plenty of space between them as they are large. You may need to bake in batches.
Step 9: Bake for 18-20 minutes, or until the edges are golden brown and set, but the centers still look slightly soft and gooey. The exact baking time may vary depending on your oven.
Step 10: Remove from the oven and let the cookies cool on the baking sheet for at least 10-15 minutes before transferring them to a wire rack to cool completely. This allows them to set up without breaking apart. Serve warm for the ultimate gooey experience.