Preheat the Oven: Preheat the oven to 350°F (180°C). Line the cookie sheets with parchment paper or silicone baking mats.
Combine the Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt until evenly combined.
Cream the Butter and Sugars: In a separate large bowl, beat the slightly softened butter, light brown sugar, and granulated sugar until the mixture is creamy and no visible clumps of butter remain.
Add the Egg and Vanilla: Beat the egg and vanilla extract into the butter mixture until the egg is fully incorporated and no pieces of egg remain visible.
Mix the Dough: Add the dry ingredients to the butter mixture in about two additions, beginning with the mixer on low speed. Mix until the ingredients come together into a cookie dough.
Fold in the Chocolate Chips: Gently fold about ¾ cup of the milk chocolate chips into the dough. Reserve the remaining ¼ cup for adding to the tops of the cookies after baking.
Portion the Dough: Scoop approximately 1½ to 2 tablespoons of dough for each cookie, forming each portion into a ball. The dough balls should be slightly smaller than a golf ball. A 1½-tablespoon cookie scoop can be used for consistent portions.
Arrange the Cookies: Place the dough balls on the prepared cookie sheets, leaving about 1½ to 2 inches of space between each one.
Bake the Cookies: Bake one sheet at a time in the middle of the preheated oven for about 9–11 minutes, or until the tops look just set.
Finish with Chocolate: Remove the cookies from the oven and, if desired, immediately dot the tops with a few of the reserved milk chocolate chips.
