Begin by creaming 1 cup of unsalted cold butter with ¾ cup of brown sugar and ½ cup of granulated sugar in a large mixing bowl. Use an electric mixer on medium speed for about 4-5 minutes until the mixture is light and fluffy. Then, add 2 cold eggs one at a time, mixing thoroughly after each addition to ensure even incorporation.

In a separate bowl, whisk together 2 ¾ cups of all-purpose flour, 1 tablespoon of cornstarch, ¾ teaspoon of baking powder, ¾ teaspoon of baking soda, and ¾ teaspoon of salt. Gradually add this dry mixture to the wet ingredients in the mixing bowl while mixing on low speed. Stop as soon as combined to keep the dough tender.

Gently fold in 1 cup of semi-sweet chocolate chips and 1 cup of milk or white chocolate chips into the cookie dough using a spatula. If you're opting for extra flavor, mix in optional add-ins like 1 cup of chopped walnuts or pecans. Ensure all ingredients are well-distributed.

Divide the cookie dough into 8-10 large balls, each weighing about 6 ounces. Place them on a tray lined with parchment paper, ensuring enough space between them. Chill the formed cookie dough balls in the refrigerator for 30-60 minutes.

Preheat your oven to 400°F (205°C). Place the chilled dough balls on parchment-lined baking sheets, spacing them apart. Bake for 9-11 minutes, watching closely as the edges become set while the centers remain slightly underbaked.

After baking, allow the cookies to cool on the baking sheet for about 10 minutes. Transfer to a wire rack to cool completely.