01 - In a small saucepan over medium heat, combine whole milk, chopped dark chocolate, cocoa powder, granulated sugar, vanilla extract, and a pinch of salt. Whisk constantly until the chocolate is fully melted and the mixture is smooth and steaming, about 5 to 7 minutes. Avoid bringing to a boil.
02 - While the hot chocolate heats, combine marshmallow creme and vanilla extract in a mixing bowl. Gradually whisk in the chilled club soda until the mixture becomes smooth and frothy.
03 - Pour the prepared hot chocolate evenly into two large mugs or heatproof glasses.
04 - Carefully spoon the marshmallow cream soda mixture over the hot chocolate, allowing it to float on the surface.
05 - Top each drink with mini marshmallows, whipped cream, and chocolate shavings as desired. Serve immediately with a straw and a spoon.