Cream egg yolks and sugar in a large bowl with an electric mixer until light and fluffy. Set aside.
Combine milk, whipping cream, and nutmeg in a large saucepan. Cook over medium heat until mixture just begins to simmer with 1 to 2 bubbles every 2 to 3 seconds, about 3 minutes. Remove from the heat.
Add a very small amount of the hot milk mixture into the egg yolk mixture while constantly whisking the eggs. Continue adding small amounts of the milk mixture, whisking until all has been added; transfer eggnog back into the saucepan. Continue to cook over medium heat, whisking constantly, until liquid reaches 160 degrees F (70 degrees C). Pour eggnog into a large pitcher, cover, and refrigerate 6 hours to overnight.
Blend cooled eggnog with an immersion blender until no lumps remain, about 1 minute. Ladle into 7 cups and top each with a dollop of whipped cream and a sprinkle of cinnamon.
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