Prepare the mugs. Drizzle chocolate syrup around the inside rim of each mug, letting it drip naturally down the sides. Set aside.
Heat the cocoa base. In a medium saucepan over medium heat, whisk together milk, cream, cocoa powder, sugar, and salt. Whisk constantly to keep the milk from scorching. Heat until smooth and steaming (but don’t let it boil).
Add the chocolate. Stir in the chocolate chips and whisk until completely melted and silky smooth. Remove from heat and stir in vanilla extract.
Pour and top. Carefully pour the hot cocoa into the prepared mugs. Add a generous handful of mini marshmallows on top.
Finish and enjoy. Dust lightly with cocoa powder for that café-style finish. Serve immediately while warm and cozy.