With a serrated knife, shave the chocolate into very small pieces. This should use about 1/5 of a standard bar of chocolate.
In a small pot over medium heat, add the milk and stir occasionally. Just before the milk boils, when small bubbles appear, add the sugar and stir with a whisk to incorporate it. Once incorporated, add the cinnamon, cayenne, vanilla and nutmeg. Slowly mix in the two chocolates, whisking continually. Once the chocolate is incorporated, add the salt and stir.
Whisk vigorously over low heat until a froth develops on the top. The froth is one of the most important parts of a good hot chocolate. Use a blender on puree for a second or two if you have one, but a whisk will do just fine. Serve in a wide mug that allows for good froth distribution. Makes one big mug or two smaller ones.