Add the butter to a medium saucepan and cook over medium heat until melted, foamy, starts making crackling noises, and deeply golden with brown flecks and a nutty aroma, stirring frequently, then immediately pour into a large heatproof mixing bowl (scraping in the brown bits) and let cool to room temp (about 45 minutes)

Once the butter has completely cooled, preheat your oven 350 and line two sheet pans with parchment paper. Set aside.

Add the brown sugar and granulated sugar to the cooled browned butter and using an electric mixer or a hand whisk, whisk until smooth and glossy and the mixture has slightly lightened up in color. About 2 minutes.

One at a time, whisk in the eggs, egg yolk, and vanilla extract and whisk for about 15 second after each addition until fully combined. The volume should start to increase and the color should be lighter!

Switch to a rubber spatula, add in the baking soda, baking powder, corn starch, salt, flour, graham cracker crumbs, and chopped chocolate and fold until evenly distributed and no dry streaks remain and the dough is soft but scoopable.

Scoop the dough into 3 tablespoon portions and gently flatten each scoop into a thick disk.

Spoon 1–2 teaspoons of marshmallow fluff onto the center of each portion, then loosely bring the dough up and around the marshmallow, rolling into a ball and leaving a small amount of marshmallow visible on top.

Place the dough balls on the prepared baking sheets, spacing them 2–3 inches apart.

Bake for 10 minutes, until the edges are set and the centers still look slightly underbaked.

Pull out the cookies, and Immediately press two pieces of chocolate bar into the top of each warm cookie so the chocolate melts into the surface.

Sprinkle lightly with flaky sea salt and more cracker crumbs if desired, then let the cookies cool on the baking sheet.