Preheat oven to 325-degrees. Line an 8x8-inch baking pan with parchment paper and spray with baking spray. Set aside.

In a microwave safe bowl, add the butter, sugar, cocoa powder and salt. Microwave in 30-second intervals, stopping to stir the mixture every 30-seconds. Microwave the mixture until the mixture is fairly smooth and hot to the touch (approximately 1-1/2 to 2-1/2 minutes in the microwave).

Remove the bowl from the microwave and set the bowl aside for several minutes, to allow the mixture to cool so it is only warm to the touch. The mixture will look gritty. After mixture has cooled for several minutes, use a wooden spoon to stir in the vanilla extract. Add the eggs one at a time to the mixture, vigorously stirring each egg into the mixture. Stir the flour into the mixture until the flour is fully incorporated into the batter. Beat the batter vigorously for 40 strokes. Fold in the graham cracker pieces, mini marshmallows and chocolate into the brownie batter. Spread brownie batter into prepared pan.

Bake brownies in 325-degree oven for 25-35 minutes. These brownies will bake anywhere from 25-35 minutes.  The variance on baking times comes from the type of pan you use (glass versus metal) and if the pan is a true 8-inch x 8-inch pan.  So start checking at the 25-minute mark and bake only until a toothpick inserted in the middle comes out without raw batter on it.  Do not over bake, or you won't get fudgy brownies.

Remove brownies from the oven and top with a few more marshmallows, chocolate chips and graham cracker pieces (if desired). Let brownies set in the pan to cool completely before slicing into desired sized squares.