Add cream, milk, nutmeg, salt, and vanilla to a small pot and bring to a simmer over medium heat.
Combine egg yolks and sugar in a separate bowl and whisk until smooth and light in color.
Once you see bubbles on the surface of the ingredients in the pot, add a small amount of the mixture to the yolk/sugar bowl, whisking constantly, to begin tempering. Repeat until the whole milk/cream mixture is added to the yolk/sugar bowl, whisking constantly.
Once all ingredients are in the yolk/sugar bowl, add back to the pot and while whisking constantly over medium heat, bring to 160 degrees, roughly 4 minutes.
Pour into a pitcher, cover with plastic wrap, and place in fridge until fully chilled.
EGGNOG FROSTING
In a stand mixer with paddle attachment, add all butter and beat on high for 4 minutes, scraping the bowl once light and fluffy.
Add 4 cups powdered sugar, eggnog, and salt to mixer and mix on low speed for 30 seconds, then increase speed to high until frosting is smooth.
Add more powdered sugar if needed to get desired consistency.