Preheat the oven to 325 degrees. Line baking sheet with parchment paper.
Combine the flour, baking soda, cornstarch and salt (omit salt if using salted butter).
In a separate bowl, whisk together the melted butter, brown sugar and white sugar.
Mix in the egg and vanilla.
Mix in dry ingredients.
Use a cookie scoop or form into balls and place on the prepared baking sheet.
Bake for 8 minutes.
Remove from the oven and place chocolate and then marshmallow on top of the cookies.
Return to the oven for 2 minutes and then broil for a minute to toast the marshmallow.
Cool for a few minutes on baking sheet and then move to a wire rack to cool.