1/4cup(1.35 ounces) roasted and salted hazelnuts, finely chopped
Center a rack in the oven and preheat to 350 (F). Spray an 8 x 8-inch baking pan with cooking spray, before lining the bottom with parchment paper and leave an overhang on two sides. Spray the parchment paper with cooking spray and set aside.
Bring 1/2 cup Guinness beer to a boil in a small, heavy-bottomed pot over medium heat. Continue boiling until the Guinness is reduced to 1/4 cup, about 5 minutes. Set aside on a wire rack to cool.
Combine 12 ounces finely chopped 100% cacao chocolate and 1 cup unsalted butter in a medium, heatproof glass bowl. Make a double boiler by placing the glass bowl on top of a sauce pot full of simmering water, ensuring that the bottom of the glass bowl does not touch the water. Allow the chocolate and butter to melt, using a heatproof rubber spatula to stir occasionally until completely melted and smooth. Remove from heat and set aside on a wire rack to cool for 5 minutes.
Transfer 1 1/2 cups granulated sugar to the bowl of a freestanding electric mixer fitted with a whisk attachment. With the mixer on its lowest setting, add 3 large eggs, one at time, only adding the next egg when the previous egg is fully incorporated. Add 1 teaspoon pure vanilla extract, then, with the mixer still on low speed, slowly pour the melted chocolate into the mixture. Once all the melted chocolate has been added, slowly pour in the reduced Guinness beer.
Turn off the mixer. Sprinkle 3/4 cup and 1 teaspoon kosher salt over the mixture and use a rubber spatula to fold the dry ingredients into the batter until just combined — be careful not to overmix! Transfer the batter to the prepared baking pan and bake for 35 minutes, or until a toothpick inserted into the center of the pan comes out with a few moist crumbs attached. Remove from oven and sprinkle 1 cup Lucky Charms marshmallows, 1/4 cup hazelnuts, and 1/4 cup mini chocolate chips on top of the brownies. Return to the oven and bake until the marshmallows are shiny and puffed, around 5 to 8 minutes.
After the marshmallows are shiny and the chocolate chips are a little bit melted, remove from the oven and allow to cool completely on a wire rack. Once the brownies are room temperature, use a serrated knife to slice 16 small squares. Wipe off the knife and/or dip the knife in warm water after each slice to prevent the marshmallows from sticking. Enjoy!