Preheat your oven to 350°F and prepare a 9-inch round cake pan by greasing it and lining the bottom with parchment paper.

Mix graham cracker crumbs, melted butter, and sugar in a bowl until well combined. Press the mixture firmly into the bottom of the prepared cake pan.

In a large bowl, beat the softened butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry ingredients to the butter mixture alternately with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.

Pour the cake batter over the graham cracker crust in the prepared pan. Smooth the top with a spatula. Bake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center comes out clean.

Place the chopped chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for a minute. Stir until smooth and glossy. Let the ganache cool slightly before using.

In a large bowl, beat the softened butter until creamy. Add the marshmallow fluff and beat until smooth. Gradually add the confectioners' sugar and vanilla extract, beating until the frosting is light and fluffy.

Once the cake has cooled completely, spread a layer of chocolate ganache over the top. Allow it to set slightly before spreading the marshmallow frosting over the ganache. Use a spatula to smooth the frosting, creating swirls for texture. If desired, use a kitchen torch to lightly toast the marshmallow frosting for an authentic campfire look.

Slice the cake into 8 pieces and serve. Enjoy the gooey combination of chocolate, marshmallow, and graham cracker in each bite.

Store any leftover cake in an airtight container in the refrigerator for up to 3 days. Bring to room temperature before serving for the best texture.