In a sauce pot over medium heat, add all of the ingredients except the egg yolk and bring to a simmer while whisking constantly. When the chocolate has melted, taste for sweetness and add more maple syrup as needed.
Place the egg yolk in a mixing bowl and spoon in a few tablespoons of the hot chocolate. Whisk until smooth, then pour the egg mixture back into the pot while stirring constantly. Bring the hot chocolate back to a simmer, then pour into 2 mugs and serve.