In a large microwaveable bowl, add the butter and chopped chocolate. Heat the mixture in 30-second intervals at 50 % power. After each interval, remove the bowl and stir the contents thoroughly. As the chocolate and butter approach a fully melted state, reduce the heating time to short bursts of 5-10 seconds. Continue until both the chocolate and butter are completely melted. Set aside to cool slightly.
Add the sugar and eggs to the bowl of a stand mixer or large mixing bowl. Beat the mixture on medium speed for approximately 5 minutes, or until the mixture has doubled in size and achieved a pale yellow color.
Turn the mixer off and pour the cooled butter and chocolate mixture, along with the vanilla extract, into the bowl. With the mixer on its lowest setting, mix until a uniform color is achieved throughout the batter. Scrape down the sides and bottom of the mixing bowl to ensure all the ingredients are completely incorporated.
In a separate bowl, add the flour, cocoa powder and salt. Whisk to combine.
Add the dry ingredients to the wet ingredients. Use a hand held spatula or wooden spoon to gently fold the ingredients together. Do not over mix.
Carefully pour the brownie batter into the prepared pan with the graham cracker crust. Use the back of a spoon or an offset spatula to smooth the top.
Place the pan on the center rack and bake for 35 minutes. To determine doneness, carefully insert a toothpick into the center of the pan. The toothpick should have moist crumbs clinging to it, but not large chunks of uncooked batter.
Remove the brownies from the oven and arrange the marshmallows in rows on top of the brownie. Return the pan to the oven for 3 minutes. At this stage, you can use a chef’s blowtorch to toast the tops of the marshmallows or set the pan under a hot broiler for 30 seconds.
Leave to cool completely in the pan before slicing into squares.
Serve and enjoy!