To make the cake, preheat oven to 350 degrees. Grease the edges of two 8-inch round cake pans. Dust with flour and tap out the excess. Line the bottom of each pan with a circle of parchment paper. Combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix until combined.
In a small bowl, combine the buttermilk, oil, eggs and vanilla. Pour the buttermilk mixture into the dry ingredients and beat just until smooth. Add coffee and mix just until incorporated. Divide the batter evenly into the prepared pans. Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean. Cool the cakes in the pans for 10 to 15 minutes, then run a knife around the edges, turn the cakes out onto a wire rack and peel off the parchment paper.
To make the frosting, place the marshmallows on a cookie sheet lined with parchment paper. Place on lower rack of oven, and broil marshmallows until nice and brown on top, between 30 to 60 seconds. Remove pan from oven and gently turn the marshmallows over, and broil until they are golden brown. (Be sure to keep an eye on them because they burn very quickly.)
In an electric mixer fitted with the paddle attachment, combine butter and confectioner’s sugar on low speed until blended, about 1 minute. Add vanilla and mix on medium-high speed for about 3 minutes. Add marshmallow cream and toasted marshmallows and mix on low speed for about 1 minute.
To assemble cake, place one cake layer on a cardboard cake round. Spread about 1/3 of the frosting over the cake. Sprinkle graham cracker crumbs and pecans on top of frosting. Top with second cake layer. Frost the top and sides of the cake with the remaining frosting. Garnish with graham cracker crumbs and shaved chocolate as desired.