In a small saucepan over medium-low heat, warm 1 cup half-and-half and 1 cup milk until it’s steaming but not boiling.
1 cup milk, 1 cup half-and-half
In another bowl, put the chopped 4 oz dark chocolate and whisk in a ladle full of hot milk mixture. Whisk in one ladle at a time until the chocolate is fully melted. By doing it like this, you are essentially making a ganache and dissolving the chocolate fully so it doesn't get grainy or separated. Keep stirring gently until the mixture becomes smooth and silky.
4 oz dark chocolate
Pour the chocolate mixture back into the remaining milk mixture. Add 2 tbsp sugar, 1 tsp pure vanilla extract, and a tiny Pinch salt. Stir until everything is dissolved and well-blended.
2 tbsp sugar, 1 tsp pure vanilla extract, Pinch salt
Serve in mugs and garnish with marshmallows. Toast with a torch for extra flavor.