In a large mixing bowl, beat together 1 cup of room-temperature unsalted butter, 1/2 cup of granulated sugar, and 1 cup of brown sugar using a hand mixer or stand mixer. Mix on medium speed for about 3-5 minutes, until the mixture is light and fluffy.

Add one large egg and 1 teaspoon of vanilla extract to the bowl. Continue mixing on low speed until fully incorporated and smooth.

In a separate bowl, whisk together 2 1/4 cups of all-purpose flour, 1 teaspoon of cornstarch, 1 teaspoon of baking powder, 1 teaspoon of baking soda, and 1/2 teaspoon of salt.

Gradually fold the dry mixture into the wet mixture, using a spatula or wooden spoon to ensure everything incorporates smoothly.

Gently fold in 2 cups of semi-sweet chocolate chips and, if desired, 1 cup of chopped walnuts until evenly distributed.

Cover your mixing bowl with plastic wrap and chill the dough in the refrigerator for at least 30-60 minutes.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Using a cookie scoop or spoon, portion out the chilled dough onto the prepared baking sheets, leaving about 2 inches of space between each scoop.

Place the baking sheets in the preheated oven and bake for 10-12 minutes, until golden around the edges.

Remove the sheets from the oven and allow the cookies to cool on the sheets for about 5 minutes before transferring to a wire rack.