Instructions
In a large saucepan, combine whole milk, vanilla bean, split and scraped (or vanilla paste), and allspice berries, lightly crushed. Heat over a medium flame, swirling occassionally, until bubbles form around the sides and the milk is steaming. Cover and steep for 20-30 minutes.
Place heavy cream and freshly grated nutmeg in a large bowl and place a fine-mesh strainer over the top.
In another large bowl or in the bowl of a stand mixer fitted with the whip attachement, vigorously beat together eggs, room temperature, egg yolks, room temperature, sugar, and salt with a sturdy wire whisk until thickened and pale, 3-5 minutes. This helps the eggnog cook faster and gives it a thicker, more volumptuous mouthfeel.
When the milk has finished steeping, heat it again until steaming, then slowly dribble the hot milk into the eggs, whisking constantly. Return the mixture to the pot and cook over medium-low heat, stirring constantly with a heat-proof silicone spatula, until the mixture reaches 165º on an instant-read thermometer, 3-5 minutes. (Or, if your milk was hot enough, you may not need to cook it at all.) Immediately strain the milk mixture into the heavy cream.
Chill the mixture over an ice water bath until cold, then stir in brandy (if using) and refrigerate until thoroughly chilled, at least 1 hour and up to 3 days.
To serve, pour into small glasses and grate a bit of fresh nutmeg over the top.