Add the almond milk and coconut milk to a small saucepan along with a generous pinch of salt. Don't worry if the fat has separated from the liquid of the coconut milk - it will all come together when it heats up.
Using a paring knife, split the vanilla bean open long ways. Using the back of the knife, run the knife along the split to scrape up the seeds. Add the seeds and bean pod to the saucepan.
Set the saucepan over medium heat and, whisking frequently, gently bring the mixture up to a bare simmer. Remove from the heat once it comes to a simmer.
While heating up the milk mixture, combine the eggs, egg yolks and sugar in a large mixing bowl. Whisk vigorously for 1-2 minutes, until the mixture is pale and very frothy.
Once the milk mixture has come to a simmer, it's time to temper the egg mixture. Whisking the egg mixture constantly, very slowly stream in the hot milk mixture. Continue whisking until all of the milk has been incorporated. Return the custard to the saucepan.
Place the saucepan over medium-low heat and, whisking constantly, slowly heat the custard until it reaches a temperature of 80-85°C (175-185°F) on an instant-read thermometer. Do not exceed this temperature as anything higher can cause the custard to break. (see note 2)
Remove the custard from the heat and pour through a fine mesh sieve set over a large mixing bowl.
Whisk in the nutmeg and the brandy, if using. Allow to cool to room temperature on the countertop before moving to the refrigerator and allowing to chill thoroughly, around 2 hours, before serving (see note 3)
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Notes
If you don't have/don't like brandy, feel free to use whiskey or light rum in its place. The eggnog is also delicious when alcohol-free.
If you don't have a thermometer, remove the custard from the stovetop when it is very steamy but has yet to come to a boil.
You can make the eggnog a few days in advance to serving. It will keep, stored in an airtight container, for up to a week.