1. Preheat oven to 350 degrees Fahrenheit. Spray a nine-inch loaf pan or two eight-inch loaf pans very well with oil spray.
2. In a large bowl, stir together sugar, flour, cocoa, baking powder, baking soda, salt, and vanilla sugar. Add eggs, water, and oil and beat for two minutes. Add boiling water and stir until combined.
3. Pour into loaf pan(s). Bake for 35–55 minutes or until a toothpick inserted in the middle comes out clean. Allow to cool.
4. Line a small baking tray with Gefen Parchment Paper. Remove cake from pan and place onto baking tray. With a sharp knife, carefully cut half-inch vertical rectangular slits. Don’t cut through to bottom—stop halfway through (this is called hasselback).
5. Melt marshmallow fluff in a microwave-safe bowl. Fill slits with melted fluff. Sprinkle crushed graham crackers, chocolate chips, and marshmallows on top of fluff.
6. Place in a broiler and broil for three minutes, until golden brown (optional).
7. Drizzle chocolate syrup over cake.