Beat the yolks to a froth (until light in color) and beat in the sugar. Slowly stir in the liquor (no sipping! You might get tipsy just smelling it!). Whip the cream and stir into the mixture. Beat the whites with clean beaters until light and foamy, but not stiff, and stir them into the mixture.
After the eggnog has set it develops a "crust". This should be cut through to quarter the "crust" and the eggnog stirred in the same direction. (because grandma said so!)
This eggnog has a different consistency and texture from most should be made at least a day ahead and kept cold. It makes a lot!