In a spacious mixing bowl, vigorously whisk the eggs and granulated sugar together until the mixture is well combined and appears slightly frothy. This should take about 2-3 minutes.
In a medium-sized saucepan, combine the whole milk and heavy cream. Heat over medium-low heat, stirring occasionally, until the mixture is warm to the touch but not boiling (roughly 5 minutes).
Carefully pour the warm milk and cream mixture into the egg and sugar mixture in a slow and steady stream, whisking constantly. This gradual incorporation is crucial to temper the eggs and prevent them from scrambling.
Once well incorporated, return the mixture to the saucepan. Over low heat, continue to stir constantly until the mixture thickens slightly, which should take about 5-7 minutes. Ensure the mixture does not come to a boil to maintain a smooth texture.
Remove the saucepan from heat and stir in the non-alcoholic amaretto syrup, pure vanilla extract, freshly grated nutmeg, and ground cinnamon until fully combined.
Allow the creamy mixture to cool to room temperature before transferring it to the refrigerator. Chill for at least 2 hours to enhance the flavors and achieve the perfect texture.
When ready to serve, pour the chilled eggnog into prepared glasses. Top each serving with a generous dollop of whipped cream and a light sprinkle of cinnamon. For an elegant touch, consider adding a cinnamon stick to each glass for garnish.