1. Preheat oven to 350°
2. Line a baking sheet with parchment paper and set aside.
3. In bowl of stand mixer beat butter and brown sugar until light and fluffy, about 2 minutes. Add in egg, extra yolk and vanilla, mix until combined, scraping sides as necessary.
4. Add in baking soda, salt and cornstarch, beating until incorporated. Finally add in the flour, mixing until dough comes together.
5. Stir in chocolate chips.
6. Drop large scoops of dough onto baking sheet and bake for 8 minutes. Remove from oven and press a marshmallow half gently on to each cookie. Return to oven for another 2 minutes until marshmallow is slightly puffed, but not toasted. Cookies should be slightly golden at the edges, but not cooked all the way through.
7. Cool on wire rack completely.

Frosting

1. Beat butter and marshmallow fluff together until smooth. Slowly add powdered sugar and mix until combined and smooth, with no lumps.
2. Spread a heaping tablespoon of frosting onto each cookie, covering the marshmallow.