Preheat the oven to 350° F and prepare a baking sheet with parchment paper.
In a stand mixer fitted with a paddle attachment, cream butter and sugar together.
Add eggs and vanilla extract. Mix until combined.
Add flour, corn starch, baking powder, baking soda, and salt. Mix until just combined.
Stir in 1 1/2 cups of marshmallows, 1 1/2 cups crushed Oreos, and sprinkles.
Place dough in the fridge for 30 minutes.
Use a large cookie scoop to scoop out cookie dough.
Bake for 11 - 13 minutes.
Place a few of the remaining mini marshmallows and Oreos on top of each cookie. Place back in the oven for 30 seconds.
Let cool on baking sheet before transferring to a wire rack.