1. In a saucepan, combine the milk, cream, cinnamon, nutmeg, and vanilla and slowly bring the mixture to a boil.
2. In a bowl, whisk the egg yolks with sugar until yolks are pale in colour. Slowly add the hot milk mixture to the egg yolks in a slow constant drizzle until combined.
3. Return the mixture to the saucepan and cook over a low heat until it thickens slightly and coats the back of a spoon but does not boil. If using a thermometer, mixture should reach 80°C (176°F).
4. Place Frangelico or other alcohol, if using, in a bowl with the white chocolate. Pour over the warm mixture and whisk until combined. Place in the fridge until chilled. This can be done two or three days ahead.
5. When ready to serve, whip the serving cream and sugar together.
6. Grate some milk chocolate onto a piece of paper.
7. Dip the rim of each glass into the whipped cream and dip in the grated chocolate. Fill each glass two-thirds of the way with eggnog.
8. Pipe or scoop the whipped cream onto the top of the eggnog and dust with ground nutmeg.