Start by melting excellent quality unsweetened chocolate (Scharffen Berger, for example) with water.
Mix in the sugar and salt. Cook for three minutes, then add the egg yolk.
Meantime, whip heavy cream until it forms stiff peaks and
fold the chocolate into the cream.
Now, here's where all you do-ahead folks will be happy. Pour the nog base into a jar and place in the refrigerator until you want to use it.
When you're ready, place a couple of tablespoons of the chocolate mixture into a cup, and pour hot milk over it while blending. You can always add in more of the chocolate to make it less milky and more chocolate-y.
Grate some fresh nutmeg over the top and voila -- Hot Chocolate Nog!
One caveat: Temper the egg yolk before adding it to the chocolate, lest you end up with scrambled egg yolk as I did (see below). If that happens, simply strain it out, get another yolk, and temper it before adding. (To temper, simply add a bit of the warm chocolate to the yolk to warm it up, preventing it from scrambling which happens when cold egg hits the hot chocolate.)
I followed the recipe exactly.