In a large bowl or bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugars together on high until light in color and texture, 5 minutes. Scrape down the sides of the bowl as needed. Add the egg and vanilla and mix well to combine.
In a medium bowl, whisk the flour, baking soda, and salt.
Add the flour mixture in 3 parts to the butter mixer, mixing on low between additions. Mix until streaks of flour still run throughout. Add the chocolate, golden grahams and marshmallows and mix until everything just comes together. You may have to use a wooden spoon to mix these last ingredients in by hand.
Refrigerate the dough for 24 hours.
When ready to bake, preheat the oven to 360° F. Line two baking sheets with parchment paper. Scoop the dough onto the baking sheets.
Bake for 10 to 12 minutes. Rotate the sheets halfway through if they appear to be baking unevenly. The cookies should be lightly golden on the outside but still look quite gooey on the inside. Let the cookies cool on the baking sheets for at least 5 minutes.
Transfer the cookies to a wire rack to finish cooling or leave on parchment and slide off of the baking pan onto a counter.
These are best eaten the day of baking but will keep, if well sealed, for up to 2 days.

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