Grease or Line two baking sheets with parchment paper.
Preheat oven to 350 degrees F (175 degrees C).
In a large mixing bowl, mix flour, cornstarch, baking soda, baking powder, and salt. Set aside.
With an electric mixer, cream butter until smooth.
Add white and brown sugar. Mix until light and fluffy.
Add eggs and vanilla extract. Mix until combined.
Add dry ingredients mixture half at a time, mixing in between additions.
Roll approx 1.5 tbsp. Of batter into balls and place onto a prepared baking sheet.
Add 3-4 mini marshmallows onto center of the cookie dough.
Add approx 1.5 more tbsp. Of batter on top of marshmallows and press down.
Roll in sprinkles. Place on baking pans and space 4-5 inches apart (6 per pan).
Flatten slightly and place mini marshmallows on top.
Bake 12-15 minutes. Enjoy!
Notes
Use marshmallow fluff for fillingInstead of using mini marshmallows for the inside of the cookies, you can use marshmallow fluff. Marshmallow fluff may spread a bit easier on top of the cookie dough.Can I use large marshmallows? Yes! You can use large marshmallows but I would recommend ripping them into pieces so they cook evenly. You can fill and top cookies with one large marshmallow ripped into pieces.