Preheat the oven to 350 degrees F. Line baking sheets with parchment paper. Set aside.
Using an electric stand mixer fitted with the paddle attachment or in a bowl with a handheld electric mixer, add the butter, brown sugar, granulated sugar, baking soda, and salt. Mix together until very light and creamy, 2-3 minutes, scraping down the sides of the bowl as needed.
Add the eggs and vanilla and mix until evenly combined.
Add the flour and graham cracker crumbs and mix until just barely combined; a few dry streaks are ok.
Add the chocolate chips or chunks and marshmallow bits and mix until incorporated.
Scoop the dough by heaping tablespoons onto the prepared baking sheets, spacing at least 2-inches apart.. I like to roll the mounds of dough so they are smooth, but you don't have to be obsessive like me.
Bake for 10-12 minutes until just golden on the edges and a little crackly on top. Let the cookies sit for a few minutes on the baking sheet before removing to a wire rack to cool completely.
Notes
Graham Cracker Crumbs: I process rectangle graham crackers in a blender until evenly ground (and no large pieces remain). Storebought graham cracker crumbs may be a bit too finely ground for this recipe (and lack the same flavor); if using, measure with a light hand and don't pack the crumbs into the measuring cup.Flattening Cookies: these cookies are on the soft and chewy, slightly thinner side of the cookie spectrum. If you often bake cookies that spread more than you like (happens a lot at high elevation), try baking the cookies on convection mode (if you have an oven with that setting) OR increasing the baking temperature to 375. It also might help to add another 1/4 cup (or more) flour to the dough for a thicker cookie.Marshmallow Bits: regular mini marshmallows will not work! They'll melt into huge craters and become a puddly, burned mess. Dehydrated mini marshmallows are key. These are often found in grocery stores near the other marshmallows, by the hot chocolate, or sometimes next to the ice cream toppings. I've also ordered them on Amazon (this brand and this brand - aff. links).Make Ahead: I've shaped this dough into balls and refrigerated the unbaked cookie dough for a day or so before baking. Works great, even with the marshmallows. I haven't tried freezing the unbaked cookie dough, but I have frozen the baked cookies several times and they are still equally delicious (frozen or thawed).