Combine eggnog and milk in a small saucepan over medium low-heat.
Heat gently, whisking frequently, until mixture is steaming and hot but not boiling (it should be about 150 degrees F).
Use a handheld frother for 3-5 seconds or whisk briskly to get a light froth.
Brew espresso into a heat-proof mug. Poured steamed eggnog over espresso.
Top with whipped cream, if desired, and a sprinkle of ground nutmeg.