It is an ancient family recipe, very secret.
It also happens to be the exact same as this one:
http://www.chow.com/recipes/10758-best-eggnog
The secret is in the liquor, I have made it with several combinations but the best so far:
Jim Beam or Evan Williams for bourbon. (I have used Makers Mark, MM46, Bookers and Woodford Double Barrel in the past. Those are better for sipping while you make it but many people commented it was too bourbon-y for them.)
Hennessey Cognac (spendy but this is were it counts is key to the recipe, VSOP or better. This year we used Black, last year it was VSOP.)
Myers Black Rum OR a spiced rum is ok as well. My friends request The Krakken when I make their batches.
I don't generally have a large party during the holidays so I don't whip the cream and egg whites for serving. Pour some in a glass with a little fresh grated nutmeg and be merry.
I might whip up an egg white this year to try it, it looks delicious in their photo.
Tips:
It has to be kept refrigerated, mine lives in a gallon jug while bulk aging (1-3 weeks to thicken). Then it can be bottled in beer bottles or whatever. One friend's wife puts it in half pint Mason jars and attaches a bow and tag for gifts.
Put at least one small container of it away for sampling during the making of the nog the following year.