In a medium bowl, with hand mixer( or in stand mixer) whip egg yolks until frothy and lemon yellow.
Slowly add powdered sugar to yolks, mixing until completely incorporated into egg yolks. Set yolks into fridge to rest for 30 minutes.
While the yolks rest, combine milk and heavy cream in a small saucepan, over medium heat.
Add pistachios and simmer for 7 minutes.
Remove saucepan from heat and let cool slightly.
Using immersion blender, puree pistachios into milk/cream until all nuts are broken down, and mixture is thick and smooth ( about 3 minutes).
Let milk/pistachio mixture cool completely.
While pistachio puree cools, whip egg whites in a small bowl until white and frothy, (don't over mix to hard peak stage).
Remove egg yolks from fridge and place a fine sieve over the bowl.
Using a wooden spoon, slowly pour the pistachio puree through sieve, stirring with the spoon to sift the mixture through the sieve faster.
After you have passed all of the liquid out of the puree and left any pieces of nut behind, remove the sieve and save the nut pulp for another use.
Slowly mix the egg whites into the yolk/pistachio mix, stirring carefully.
Add the nutmeg and stir to incorporate.
Serve this pistachio eggnog with a sprinkle of nutmeg on top for a festive garnish!