Preheat the oven to 325 degrees. Line an 8x8 inch baking pan with parchment paper, leaving enough overhang on the sides of the pan to make for easy transfer when the brownies are done.
Make the Crust: In a mixing bowl combine graham cracker crumbs, salt & ground cinnamon. Give the ingredients a quick stir to mix them together. Add in the melted butter and mix the ingredients together until the crumbs are evenly coated. Press the mixture into the baking pan and firmly smooth it out to form a crust.
Make the Brownie Batter: in a microwave safe bowl combine chocolate chips, butter and espresso powder. Place in the microwave and heat for 20-30 second intervals, stirring the ingredients together in between each heating. Repeat steps until chocolate is thoroughly melted and smooth. Set aside to cool for a few minutes.
In a separate, large mixing bowl combine both sugars, vanilla extract and eggs. Using a handheld mixer beat the sugar and eggs on med high speed for at least 5 minutes, until the sugar and eggs are fluffy and pale in color. Do not skip on time. After 5 minutes reduce the speed to low and add in the vegetable oil, mixing until combined. Next, add in the melted chocolate and continue mixing on low speed until the chocolate is thoroughly incorporated into the batter. Fold in the dry ingredients; flour, cocoa powder & salt. Fold just until combined, do not over mix batter.
Pour brownie batter over the graham cracker crust and smooth it out evenly with a spatula. Bake for 30-40 minutes, or until a toothpick in the center comes out clean.
Allow brownies to cool for at least 45 minutes before lifting them out of the pan so they have time to set and firm up. Once they're cool transfer them from the pan onto the counter and spread the marshmallow fluff on top. Be patient as the marshmallow fluff is very sticky. Cover the entire surface of the brownies.
*Optional* use a kitchen torch to lightly char the marshmallow fluff or you can use the oven broiler and return the brownies to the oven for a minute or two.