Step 1: Preheat your oven to 400°F (200°C). Line two large baking sheets with parchment paper or silicone baking mats. This prevents sticking and ensures even baking.
Step 2: In a medium bowl, combine all the dry ingredients: all-purpose flour, cornstarch, baking powder, baking soda, and sea salt. Whisk them together thoroughly until well combined. Set aside.
Step 3: In a separate large bowl, using an electric mixer, cream together the softened butter, packed brown sugar, and granulated sugar. Whisk on medium speed until the mixture is light, fluffy, and smooth, about 2 minutes.
Step 4: Add the eggs one at a time to the butter mixture, beating well after each addition. Stir in the vanilla extract until just combined.
Step 5: Gradually add the dry ingredient mixture to the wet ingredients. Mix on low speed until just combined. Be careful not to overmix; stop as soon as no streaks of flour remain.
Step 6: Fold in the coarsely chopped walnuts and semisweet chocolate chips using a spatula until evenly distributed throughout the dough.
Step 7: Scoop generous amounts of dough onto the prepared baking sheets. For giant cookies, use a large ice cream scoop or measure out about 1/4 cup of dough per cookie. Place cookies about 3-4 inches apart, as they will spread significantly.
Step 8: Bake in the preheated oven for 15-18 minutes, or until the edges are golden brown and the centers are still slightly soft. Baking time may vary depending on your oven and cookie size.
Step 9: Remove the baking sheets from the oven and let the cookies cool on the sheets for about 5-10 minutes. This allows them to set up and prevents breakage.
Step 10: Carefully transfer the cooled cookies to a wire rack to cool completely. Enjoy warm or at room temperature!