In the bowl of a stand mixer, cream together the butter, dark brown sugar, and granulated sugar until creamy and smooth, 2 minutes. Add in the egg, followed by the egg yolk, and the vanilla, mixing well after each addition. Lastly, add in the salt, baking soda, cornstarch, flour, and 1/2 cup of ground graham cracker crumbs and mix until combined. Stir in 1 cup of the chocolate chips and 1 cup of the miniature dehydrated marshmallows and mix until incorporated. Cover and refrigerate for 2 hours, up to 24 hours; this is mandatory!

Preheat oven to 350 degrees F. While oven preheats, allow cookie dough to soften for about 5-10 minutes. Line 2 baking sheets with parchment paper or silicone liners. Using a cookie dough scoop, scoop out Tablespoonfuls of cookie dough and place 2" apart on the prepared baking sheets.

Bake for 5 minutes, then rotate pans. Bake for another 4 minutes. Remove from oven but keep oven on. Place halved mini marshmallows and chocolate chips into cookies. Return to the oven and bake for an additional 2-3 minutes or until the marshmallows are toasty and puffy and cookies are set. Remove from oven and sprinkle the tops with a little pinch of graham cracker crumbs. Cool COMPLETELY on the baking sheets before removing to a wire rack or storing cookies.

Can be stored covered at room temperature for up to 5 days.