Toss together flour, baking soda, cornstarch and salt in a large bowl. Set aside. Measure ½ cup chocolate chips and place in small bowl. Set aside.
In a medium size bowl, whisk the melted butter, brown sugar, and white sugar together until no brown sugar lumps remain. Whisk in the egg, then the egg yolk. Finally, whisk in the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. The dough will be very soft, yet thick.
Fold in the remaining 1 cup chocolate chips. They may not stick to the dough because of the melted butter, but do your best to have them evenly dispersed among the dough. Cover the dough and chill for 30 minutes. You can skip this step if you're in a hurry. Chilled dough will make your cookie a bit thicker when baked.
Preheat the oven to 350°F. Line two large baking sheets with parchment paper or silicone baking mats. Set aside.
Roll the dough into balls, about 2 tablespoons of dough each. Or use a cookie scoop to form the balls of dough. Place balls of dough 2 inches apart on cookie sheets. Use the remaining chocolate chips, press a handful of chocolate chips on top of the dough balls for dramatic effect. Bake the cookies for 11-14 minutes or until light golden brown on the edges. Allow to cool on the cookie sheet for 10 minutes before moving to a wire rack to cool completely.
Cookies stay soft and fresh for 7 whole days at room temperature (if you can keep them from being eaten for that long). Cookies may be frozen up to 3 months. Rolled cookie dough may be frozen up to three months and baked in their frozen state for 12 minutes.
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Notes
Don't have time to chill the dough? You can skip this step if you're in a hurry. Chilled dough will make your cookie a bit thicker when baked.