1 and ½cups(255g) chocolate chipsI prefer 1 cup milk or semi-sweet chocolate chips and ½ cup mini semi-sweet chocolate chips
In a large bowl, toss together the flour, baking soda, cornstarch and salt. Set aside.
In a medium sized bowl, whisk together the melted butter, brown sugar, and granulated sugar. When sugar clumps disappear, whisk in the egg, egg yolk, and vanilla.
Pour wet ingredients into dry ingredients and mix together with a large spoon or rubber spatula. Fold in the chocolate chips. Cover dough and chill in the refrigerator for at least 2 hours, or up to 3 days.
When you are ready to bake the cookies, preheat the oven to 350ºF (177ºC). Line two large baking sheets with parchment paper or silicone baking mats and set aside.
Roll the chilled dough into balls-- 1 ounce (approximately 1 Tablespoon) of dough each-- and place 8 balls of dough per cookie sheet. If you like, you can press a few more chocolate chips on top/sides of the dough balls for aesthetic purposes. Keep dough you are not rolling or baking in the refrigerator.
Bake the cookies for 10-11 minutes or until edges just start to turn brown. Remove from oven and allow cookies to cool on the baking sheet for 5 minutes before moving to a wire rack to cool completely. Store cookies in an airtight container at room temperature, up to 7 days. Baked cookies may be frozen up to 3 months. You may also freeze rolled cookie dough up to 3 months. Bake frozen for 11-12 minutes.
*Room temperature eggs: these are preferred for even distribution into batter. As a rule of thumb, when a recipe calls for room temperature or melted butter, it's generally a good idea to use room temperature eggs as well. If you don't have time to let eggs come to room temperature, place them in a bowl of warm water for 5-10 minutes before using.