Prepare the pan:
Position a rack in the center of the oven and heat to 325F degrees for dark non-stick bundt pan (350F degrees for regular metal bundt pan).
Mix together melted 2 Tablespoons of butter and 2 Tablespoons cocoa powder. This will create a thick paste. Using a pastry brush, cover the entire inside of the bundt pan. Set aside.
Make the cake:
If you're wanting to add graham crackers to what will be the bottom of the cake once inverted, make the graham cracker topping now. Otherwise, proceed in this order.
In a small saucepan, combine 1 cup butter, ⅓ cup cocoa powder, 1 teaspoon salt, and 1 cup water and place over medium heat. Cook, stirring, just until melted and combined. Remove from the heat and set aside.
Place 2 cups flour, 1 ¾ cups sugar, and 1 ½ teaspoons baking soda in a large bowl and whisk to blend. Add half of the melted butter mixture and whisk until completely blended (the mixture will be quite thick). Add the remaining butter mixture and whisk until combined. Add 2 eggs, one at a time, whisking until completely blended before adding the next egg. Whisk in ½ cup sour cream and 1 teaspoon vanilla until smooth. Gently fold in 1 cup chocolate chips.
Scrape the batter into the prepared pan and bake until a toothpick inserted into the center of the cake comes out clean, 40 to 45 minutes. Let the cake cool in the pan for 15 minutes and then invert onto a rack. Let cool completely before glazing.
Make the chocolate glaze:
Place 6 ounces bittersweet chocolate in a medium microwave-safe bowl, adding ⅔ cup heavy cream. Heat the mixture in the microwave in 45 second intervals, stirring in between cooking until the chocolate melts completely and the mixture is smooth. Stir in 1 teaspoon vanilla extract. Set aside until the cake is completely cooled. Stir again just before drizzling over the cake. If chocolate has hardened too much for drizzling, just reheat again for 10 seconds.
To make the marshmallow glaze:
Place 1 cup marshmallow creme, 1 teaspoon vanilla and 1-2 Tablespoons boiling water together in a small bowl and stir until the mixture reaches a pourable consistency. I like mine a little thick. If you want a thinner glaze, add an additional Tablespoon of water.
To make the graham cracker topping:
Place 1 ½ cups graham crackers, 6 Tablespoons melted butter and ⅓ cup granulated sugar in a bowl and stir until combined and all butter has been absorbed.
Once cake has cooled:
Here is where you can get creative and add as many or as few toppings as you like. But if you're going all out for a true gooey S'mores chocolate cake, beginning with the chocolate, drizzle over the cake, letting the glaze run down the valleys in the bundt ridges. Then do the same with the marshmallow glaze. Top with graham cracker crumbs and finish with sliced almonds or walnuts, or any additional toppings.
Store cake lightly covered for up to 2 days.