Pour 4 cups of whole milk and 1 cup of heavy cream into a medium saucepan. Add 1/2 teaspoon ground nutmeg and 1/4 teaspoon ground cinnamon. Warm the mixture over medium heat, stirring occasionally, until it’s hot but not boiling—about 5 to 7 minutes.
While the milk mixture is warming, crack 4 large eggs into a mixing bowl. Add 3/4 cup granulated sugar and whisk vigorously until the mixture is pale and slightly thickened.
Slowly pour about one cup of the hot milk mixture into the egg-sugar mixture while whisking constantly to temper the eggs and avoid scrambling.
Pour the tempered egg mixture back into the saucepan with the remaining milk and cream. Cook over low heat, stirring constantly, until the mixture thickens slightly and coats the back of the spoon—around 5 to 8 minutes.
Remove the saucepan from heat. Pour the eggnog through a fine mesh strainer into a clean pitcher or bowl to remove any cooked egg bits. Stir in 1 teaspoon vanilla extract.
Cover and refrigerate for at least 2 hours to let it cool and thicken further. Before serving, stir well and garnish with a sprinkle of nutmeg or crispy turkey bacon if desired.
