Preheat the oven to 325°F and line a 9 x 9-inch baking pan with parchment paper on all sides. Place your graham crackers in a single layer along the bottom of the pan. You may have to break your graham crackers into quarters to get everything to fit, but it doesn’t have to be perfect! Set aside.
In a heavy-bottomed saucepan over medium-low heat, melt the butter and finely chopped chocolate stirring constantly, until smooth and fully melted. Once melted, take off the heat to cool slightly.
In a stand mixer fitted with the whisk attachment, beat the sugar and eggs together until. fluffy and pale yellow in color. Scrape down the bowl and pour in the chocolate and butter mixture. Beat until homogenous. Scrape down the sides and add the vanilla. Mix well. The mixture should be glossy.
Add the flours, baking powder, and salt and mix until just combined and no streaks of flour remain. Do not overmix.
Use a rubber spatula to spoon the batter over the graham crackers in your prepared pan, spreading the batter into an even layer all the way to the sides.
Bake for 25 to 30 minutes or until a knife inserted into the center comes out mostly clean and the edges are set. Transfer to a wire rack to cool.
Once the brownies are completely cooled, spoon the marshmallow fluff over the brownies, using a spoon to spread the fluff all the way to the edges, creating big swirls.
If desired, use a blowtorch to toast the marshmallow fluff. If you don’t have a blowtorch, you can also place the brownies in the oven under the broiler for a minute or two, keeping an eye on it.
Place in the fridge for an hour to set up before slicing into 9 large brownies or 16 small brownies, making sure to wash your knife in between slices. Enjoy!