Preheat oven to 350 degrees and line a baking sheet with a silicone, non-stick baking liner or parchment paper. Set aside.
In a medium sized bowl combine flour, cornstarch, baking powder, baking soda and salt. Set this aside, too.
In a separate bowl, cream butter for 1-2 minutes until nice and soft.
Add sugar and brown sugar to butter, creaming again for ~2 minutes. Be sure to scrape the edges so it's all incorporated and there aren't any large butter chunks.
Now add in egg, egg yolk, and vanilla. Continue to mix until combined well.
Slowly add in your flour mixture to the butter mixture, 1/3 at a time. Your dough will be thick, this is good!
Now fold in your chocolate chips.
Take ~2 tbsp of dough, form a ball, and then flatten. Place ½ of a large marshmallow in the middle and then fold the sides up to cover it. You’ll want to leave a little peeping out. Ideally in a couple places VS one spot. Place the balls of dough on the cookie sheet ~2” apart. You’ll make ~18 cookies.
Sprinkle finished cookie dough balls with sea salt if desired!
*If you’re going to be leaving time between the making of the dough and baking OR if it’s really warm place your dough in the fridge.*
Bake for 12 min at 350.
Remove from oven when cookies are golden brown and the marshmallows look just a touch toasted. Allow to cool and enjoy!