Preheat the Oven: Position an oven rack in the middle of the oven and preheat to 325°F (162°C). Line the bottom and sides of an 8-inch square baking pan with parchment paper, leaving an overhang on opposite sides to help lift the brownies out of the pan after baking.
Melt Butter: Add the butter to a medium saucepan. Place over medium-low heat and cook until the butter melts completely, then turn off the heat. Stay close and do not let it brown.
Combine Ingredients: While the butter is still hot, stir in the sugar, cocoa powder, vanilla, and salt. Mix well until blended. At this point the batter will look gritty. Set the saucepan aside to cool until the mixture is warm, not hot, which will take a couple of minutes. Test the temperature by touching the batter; it should feel warm but not hot to the touch.
Add Eggs: Add the eggs one at a time, stirring vigorously after each addition. Continue until the batter is smooth and well-blended.
Add the Flour: Once the batter appears thick and well-blended, add the flour. Use a spoon to mix the flour into the batter until it becomes very thick and starts to pull away from the sides of the bowl.
Pour in the Pan: Spread the thick brownie mix evenly in the prepared pan. Because the batter is so thick, it can be challenging to spread, but do your best to push it to the corners and smooth the top.
Bake: Bake the brownies until the edges look dry and the center is slightly underbaked, about 20 to 30 minutes. In my oven they took about 24 minutes. A toothpick inserted into the center should come out somewhat moist with batter. As the brownies cool, they will firm up but remain moist and fudgy in the middle.
Add the Marshmallows: Add the homemade marshmallows across the top of the brownies.
Add the Graham Crackers: Add the graham cracker pieces between the marshmallow layer, leaving some openings for the chocolate.
Add the Chocolate: Add the pieces of dark chocolate between the marshmallows.
Broil: Broil the easy s'mores brownies for about 1 to 2 minutes until you see melted chocolate and gooey toasted marshmallows. You may need to go a bit longer depending on your broiler, just keep an eye on it as nobody likes a charred marshmallow! This can also be achieved with a kitchen torch.
