Step 1: Preparing the Dough

1. Whisk the Dry Ingredients: In a medium bowl, whisk together the flour, cornstarch, salt, and baking soda.
2. Cream the Butter and Sugars: In a large bowl, combine the cold butter, granulated sugar, and brown sugar. Use a mixer or a wooden spoon to blend until the mixture is light and creamy.
3. Add the Wet Ingredients: Crack in the egg and add the vanilla extract. Beat until fully combined.
4. Incorporate the Dry Ingredients: Gradually add the dry mixture to the wet mixture, stirring just until the dough comes together. Avoid overmixing to keep the cookies tender.

Step 2: Adding the Chocolate Chips

1. Fold in the Chips: Gently stir in the semi-sweet chocolate chips. If you’re using walnuts or other mix-ins, add them now.
2. Chill the Dough: Cover the dough and refrigerate it for at least 30 minutes to enhance the flavor and prevent excessive spreading during baking.

Step 3: Shaping and Baking

1. Preheat the Oven: Set your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Shape the Cookies: Scoop tablespoon-sized portions of dough and roll them into balls. Arrange them on the baking sheet, leaving about 2 inches of space between each ball.
3. Bake: Place the baking sheet in the oven and bake for 8–10 minutes. The edges should be set, but the centers can remain slightly soft.
4. Cool: Allow the cookies to cool on the baking sheet for 2 minutes before transferring them to a wire rack to cool completely.