Whisk together crushed graham crackers, flour, cornstarch, baking soda and salt in a medium bowl.
In the bowl of a stand mixer, cream butter and brown sugar together with a paddle attachment until light and fluffy, about 2-3 minutes at a moderate speed.
Add eggs one at a time, mixing well after each addition. Stir in vanilla.
Turn mixer to lowest setting and add dry ingredients, mixing until just barely combined. Add chocolate chips, marshmallow bits and mini marshmallows and gently mix to incorporate.
Scoop dough into cookie dough balls (I use my hands) that weigh about 100g each. Do not flatten or roll the dough into smooth balls – the more nooks and crannies, the better! Place in the fridge to chill for at least one hour, but up to 48 hours, before baking.**
Preheat the oven to 350' (325' convection). Bake cookies, 6 at a time, on a parchment or silpat-lined baking sheet for 12-15 min, until lightly golden on top and bottom***
Let cookies sit on baking sheet for 10 minutes to set. You can use a spatula to gently nudge edges of cookies into a circular shape while cookies are still warm. Transfer to a wire rack to finish cooling.
Notes
*Kerrygold salted butter can be subbed with another European-style, cultured butter with a high butterfat percentage. I just happen to be a big fan of Kerrygold. It can be bought in packs of 4 at Costco.
**If dough will be in the fridge for longer than an hour, place in a sealed container or cover with plastic wrap. I like to place scooped cookie dough balls on a baking sheet into the freezer to harden, then transfer to a freezer ziplock bag or airtight container and store frozen to have cookies on hand. Dough will keep in the fridge for 2-3 days or in the freezer for up to 3 months.
***Dough should be baked from refrigerated temperature. If cookie dough is frozen, pull out of the freezer and let sit on the counter for 15 minutes while oven preheats to warm up slightly. You can also transfer frozen cookie dough to fridge 12-24 hours before baking.