In a medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt.
In a large bowl, combine melted butter, light brown sugar, and granulated sugar. Use an electric mixer or whisk by hand until smooth and glossy.
Beat in the whole egg and extra egg yolk, followed by the vanilla extract. Mix until fully incorporated but do not overbeat.
Gradually add the dry ingredients to the wet mixture. Fold gently with a spatula until no flour is visible. Avoid overmixing.
Stir in the miniature semisweet chocolate chips, ensuring even distribution.
Cover the dough and refrigerate for at least 30 minutes to an hour to chill.
Preheat oven to 350°F (175°C). Line baking sheet with parchment paper or silicone mat. Scoop dough about 2 inches apart.
Bake for 10–12 minutes until edges are set but centers look slightly underdone.
Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
