In a medium saucepan, bring the milk to a boil. Remove from the heat and add the chocolate. Let it rest for about 2 minutes to allow the chocolate to melt.
Stir until all of the chocolate has melted and the mixture is smooth. If needed, you can use an immersion blender to fully blend. Serve hot, sprinkled with fleur de sel, and topped with marshmallows, if you like.
Chef’s Note: For a rich and decadent hot chocolate, replace 1 1/2 cups of the milk with heavy cream or half-and-half.