01 - In a medium saucepan over medium heat, combine whole milk and heavy cream. Heat until steaming but not boiling.
02 - Add chopped dark chocolate, unsweetened cocoa powder, sugar, vanilla extract, and a pinch of salt. Whisk continuously until chocolate is fully melted and the mixture is smooth and velvety, about 3 to 5 minutes. Adjust sugar to taste. Remove from heat.
03 - In a cold mixing bowl, combine marshmallow creme, cold heavy cream, and vanilla extract. Beat with an electric mixer on medium-high speed until soft peaks form and the mixture is light and airy.
04 - Pour the hot chocolate into mugs or heatproof glasses, filling each about two-thirds full.
05 - Spoon a generous dollop of marshmallow cream float over each mug.
06 - Garnish with chocolate shavings or a dusting of cocoa powder as desired. Serve immediately.