Instructions

1. Line two baking sheets with parchment paper and set aside.
2. Place half the butter (1 stick, 8 tablespoons) in a medium skillet. Melt the butter over medium heat, swirling it in the pan occasionally. It’ll foam and froth as it cooks, and start to crackle and pop. Once the crackling stops, keep a close eye on the melted butter, continuing to swirl the pan often. The butter will start to smell nutty, and brown bits will form in the bottom. Once the bits are amber brown (about 2 1/2 to 3 minutes or so after the sizzling stops), remove the butter from the burner and immediately pour it into a small bowl, bits and all. This will stop the butter from cooking and burning.  Allow it to cool for 20 minutes to 1 hour until mostly room temperature once again. 
3. Beat the remaining butter (1 stick, 8 tablespoons) with the brown sugar for 3 to 5 minutes, until the mixture is very smooth.
4. Add the cooled brown butter into the bowl, along with the granulated sugar. Beat for 2 minutes, until smooth; the mixture will lighten in color and become fluffy.
5. Add the egg and egg yolk, and beat for one minute more.
6. Add the flour, salt, and baking soda, beating on low speed just until everything is incorporated.
7. Use a spatula to fold in in the marshmallow bits, chocolate chunks, and graham cracker, and finish incorporating all of the dry flour bits into the dough.
8. Scoop the dough onto a piece of parchment paper, waxed paper, or plastic wrap. Flatten it slightly into a thick disk, and refrigerate for at least 30 minutes. About 15 minutes before you’re ready to begin baking, place racks in the center and upper third of the oven and preheat your oven to 350°F.
9. Scoop the dough in 2 tablespoon-sized balls onto the prepared baking sheets. Leave about 2 inches between the cookies; they’ll spread as they bake.
10. Bake the cookies for 12 to 15 minutes, until they’re golden brown. Remove them from the oven, and allow them to rest on the baking sheet for at least 5 minutes before moving them.
11. Serve warm; or cool completely, and store airtight at room temperature for several days. For longer storage, wrap well and freeze.