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.
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). For the graham cracker crumbs: I usually throw the rectangle graham crackers in my blender and pulse until fairly finely crushed, but I've also used the ol' ziploc bag and rolling pin method with no issues. I think the storebought graham cracker crumbs might be a bit too finely crushed and cardboardy for this recipe, but I haven't tried them myself. Tips to stop cookie spreading: 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. If the cookies still spread (or to preempt the issue), a couple extra tablespoons flour might help. About those marshmallow bits: your average mini marshmallows will not work! They'll melt into huge craters and become a puddly, burned mess ruining one of summer's most perfect cookies. Trust me, I tried it. Instead, you need marshmallow bits. In other words, dehydrated marshmallows. They should be pretty easy to find. They are often located right by the other marshmallows in the grocery store or by the hot chocolate (another great way to use them, btw), and someone mentioned looking by the ice cream toppings. I've also ordered them on Amazon (this brand and this brand - aff. links) and found them in bulk at my local Cash and Carry.