In a large bowl, stir together the flour, baking soda, salt, and cornstarch. Set these dry ingredients aside.
In a medium-sized bowl, microwave the butter to melt it. Allow the butter to cool to room temperature. Add the brown sugar with a wooden spoon, stirring to dissolve. Stir in the white sugar. Add the egg and egg yolk along with the vanilla, whisking to combine.
Pour the butter-egg-and-sugar mixture into the bowl holding the flour mixture. Stir again with a wooden spoon. The resulting mixture will be soft and thick. Stir in the chocolate chips, stirring to combine.
Put a lid on the bowl or cover it with plastic wrap. Put the bowl of dough into the refrigerator and chill it for two or three hours. Don’t skip this step or the baked cookies will spread too much! The chilling helps the butter firm up.
Preheat the oven to 325 degrees.
Use a cookie scoop or a tablespoon to scoop out walnut sized globs of dough and use your hands to roll the dough into balls. The dough may be a bit crumbly but just do your best.
Place the cookie dough balls onto a large baking sheet, at least 2 or 3 inches from one another.
Bake the cookies for 10 to 13 minutes before you remove the pans from the oven. Do not worry if the cookies look a bit too sticky and under baked. They will firm up as they cool. Let the cookies cool on the baking sheet for about ten minutes. Scrape the cookies off the pan with a spatula to a wire rack to cool completely.
Store cookies in an airtight container for up to a week.