• Preheat the oven to 350°F. Grease the bottom of an 8x8-inch pan and line it with parchment paper.

Brown butter

• Optional step. Skip if not browning the butter. Your butter does need to be hot either way. In a small saucepan, brown the butter. When the butter is a golden amber color and gives off a nutty aroma, remove it from the heat and weigh it using a food scale. It should weigh about 273 grams (or the equivalent of about 19 tablespoons). This is because some of it will evaporate when cooking.
  3 sticks unsalted butter
• Set about 91g (6.5 tablespoons) aside for the graham cracker crust. Let it cool.
• Take the remaining 182 grams (about 13 tablespoons) of hot butter and pour it directly over chopped chocolate, cocoa powder, and optional espresso powder in a container. Stir to melt the chocolate completely. Set aside to cool.
  3 ounces bittersweet or dark chocolate bar, ⅓ cup cocoa powder, 2 teaspoons instant espresso, 12.5 tablespoons brown butter, hot

Graham cracker crust

• In a mixing bowl, mix brown butter, brown sugar, vanilla extract, and the egg yolk together until smooth and combined.
  6.5 tablespoons brown butter, melted and slightly cooled, 9 tablespoons brown sugar, 2.5 teaspoons vanilla extract, 1 large egg yolk
• Add the graham cracker crumbs, all purpose flour, and salt to the wet ingredients. Mix to combine, it will have a sand-like texture that clumps together slightly.
  ¾ cup all purpose flour, ¾ cup graham cracker crumbs, ¾ teaspoon kosher salt
• Press the crust into the pan in an even layer using a cup or just your hands. Use a fork to poke holes all over the bottom of the crust. Bake for 12-15 minutes until the crust is golden brown and set. Place on a cooling rack while you work on the brownie filling.

Brownies

• In a mixing bowl, mix together the brown sugar, eggs, and egg yolk until pale and fluffy. Next, add in the vanilla extract, then, stream in the cooled chocolate, cocoa powder, butter, and espresso powder mixture until completely combined. 
  1 ¼ cups brown sugar, 2 large eggs, 1 large egg yolk, 1 tablespoon vanilla extract
• Add in the all purpose flour and salt, and switch to a rubber spatula to gently mix. Once the dry ingredients are almost all the way combined (there will still be a few streaks of flour), fold in the chocolate chunks or chips. 
  1 teaspoon kosher salt, ¾ cup all-purpose flour, ½ cup chocolate chunks

Assemble and bake

• Pour the brownie batter over top of the graham cracker crust, and spread it evenly with an offset spatula. 
• Bake for roughly 30-35 minutes, testing for doneness by inserting a toothpick into the center of the brownies beginning at 30 minutes. You’ll know they’re done when the edges are set, but a toothpick comes out with a few moist crumbs on the toothpick. Set aside to cool for about 45 minutes to an hour.

Toasted marshmallow topping

• Once the brownies have cooled off, preheat the broiler to high. Spread the marshmallow fluff over top of the brownies using an offset spatula.
  7.5 ounces marshmallow creme
• Place under the broiler for 3-5 minutes, watching VERY CAREFULLY to prevent the marshmallows from burning. TRUST US, it goes very quickly!
• When the top is nice and golden brown, set it aside to cool for about 15-20 minutes before cutting in! This allows the topping a bit of time to set up.

Serve

• Now, all that's left to do is cut these brownies into 12-16 equal pieces and enjoy!
• Store leftovers in an airtight container at room temperature for up to 5-6 days. To freeze, wrap tightly in plastic wrap, then aluminum foil, and place in the freezer for up to 3 months