Step 1: Prepare the Graham Cracker Crust

1. Preheat your oven to 350°F (175°C).
2. In a bowl, mix graham cracker crumbs, sugar, and melted butter until combined.
3. Press the mixture into the bottom of a greased 9-inch cake pan.
4. Bake for 10 minutes, then set aside to cool.

Step 2: Make the Chocolate Cake

1. In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
2. Add eggs, milk, oil, and vanilla. Mix until combined.
3. Slowly pour in boiling water while stirring. The batter will be thin this is normal.
4. Pour the batter over the cooled graham cracker crust.
5. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
6. Let the cake cool completely.

Step 3: Prepare the Marshmallow Topping

1. If using marshmallow fluff, skip to step 3.
2. In a saucepan, heat marshmallows and heavy cream over low heat, stirring until smooth.
3. Let it cool slightly, then spread evenly over the cooled cake.
4. Sprinkle powdered sugar on top.