In a medium saucepan, combine the bittersweet chocolate and water. Heat over medium-low heat, stirring frequently until the chocolate has fully melted, about 3-5 minutes.

Stir in granulated sugar and a pinch of salt to the melted chocolate mixture. Increase the heat to medium and bring it to a gentle boil, stirring occasionally for 2 minutes.

Lower the heat slightly and stir in both heavy whipping cream and half and half. Keep stirring until the mixture is scalding hot, about 5 minutes.

Remove from heat and use a hand mixer or milk frother to blend for about 30 seconds for a frothy texture.

Pour hot chocolate into mugs and top with whipped cream or marshmallows.

Let any leftovers cool before storing in the refrigerator for up to 4 days.