Crush the grahams: Place 2 graham crackers in a zip-top bag and crush with a rolling pin until sandy.
Pour onto a small plate.
Rim the glass: Drizzle a little chocolate syrup onto another plate. Dip the rim of a tall glass into the syrup, then into the crushed grahams to coat. Set aside.
Add chocolate base: Squeeze 1–2 tablespoons of chocolate syrup into the bottom of the glass.
You can swirl a little up the sides for flair.
Ice it up: Fill the glass with ice, about three-quarters full. This chills the cola fast and protects the graham rim from melting too quickly.
Pour the cola: Add 10–12 ounces of cola slowly to reduce foam. Leave about an inch of space at the top.
Flavor boost: Stir in 1–1.5 tablespoons of marshmallow syrup and a tiny splash (1/8 teaspoon) of vanilla extract if using.
Taste and adjust sweetness.
Creamy finish: Gently pour in 2–3 tablespoons of vanilla cream or half-and-half. Don’t over-stir—let it marble for that classic dirty soda look.
Toast a marshmallow (optional but epic): Carefully toast a marshmallow over a stove flame, kitchen torch, or broiler until golden and gooey. Skewer it and rest it on top of the glass.
Final touch: Sprinkle a pinch of crushed grahams over the top and add a chocolate drizzle.
Serve with a straw and enjoy immediately.