Whisk the flour, salt, baking powder, baking soda, and cornstarch together in a large bowl. Set it aside.
Using a stand or hand mixer, beat the butter and sugars in a second large bowl for about 2 minutes or until the ingredients are smooth and creamy.
Add the egg to the bowl with the butter and sugar, and beat the ingredients again. Then, add the egg yolk and vanilla, and beat until they are just combined. Scrape the edges of the bowl, and beat once more.
Gradually add the dry mixture to the bowl with the butter mixture, beating continuously on low speed until the ingredients are just combined.
Reserve a handful of the torn marshmallows, and set them aside. Add the remaining marshmallows, chocolate chips, and graham crackers to the dough. Beat the ingredients on low until they are just combined. Be careful not to overmix!
Refrigerate the dough for at least 1 hour.
Heat the oven to 350° F.
Once chilled, portion the dough into ¼ cup-sized pieces. Roll each piece between two hands to shape them into balls.
Place the balls of dough on a parchment-lined baking sheet 2 inches apart. Gently press in the remaining marshmallows to the dough.
Bake the cookies at 350° F for 12-14 minutes.
Remove the baking sheet from the oven. Sprinkle the remaining marshmallows on top while they are still warm, pressing them genlty into the cookies.
Leave the cookies on the sheet to cool completely.