In a large bowl add the melted and cooled butter, granulated sugar and brown sugar and beat with a hand mixer or stand mixer for about 2 minutes.
Add in the egg, egg yolk and vanilla and continue to mix until smooth and creamy.
Place a large sieve over the bowl and add the flour, baking powder, cornstarch and salt. Use a spoon to work the dry ingredients through the sieve and into the wet ingredients below.
Use a spoon to mix the dry ingredients into the wet ingredients just until blended. Then add in the chocolate chips and most of the marshmallows reserving a 1/2 cup of marshmallows for later. Mix until evenly distributed.
Cover the cookie dough with plastic wrap or a tight fitting lid and place in the fridge to cool overnight or for 8 about hours and not more than 72 hours.
When ready to bake remove the cookie dough from the fridge and let it come to room temperature or warm enough to scoop into balls. Preheat the oven to 350 F and then scoop rounded tablespoons of cookie dough and place on a parchment lined baking sheet.
Bake for 12-15 minutes removing them from the oven about 2 minutes before they are done. When you remove the cookies before baking is ended add a few of the remaining marshmallows to the top of the cookies and then place bake in the oven to finish baking.
When the cookies are golden brown remove them from the oven and let them rest on the cookie sheet for a few minutes before transferring to a cooling rack.
Store in a tightly sealed container for up to 5 days or freeze for up to 3 months.
Storage:Store baked cookies after they have cooled in a tightly sealed container on the counter for up to 5 days or freeze for up to 3 months.Tip:You don't need to wait to make these if you need a cookie right away. You can reduce the resting time in the fridge to an hour so allow the cookie dough to cool before baking. Or if you wish to bake right away but be aware this may cause the cookies to spread a little thinner than usual.