In a medium bowl, whisk together the 6 egg yolks and 200g (1 cup) of granulated sugar until the mixture is pale yellow and slightly thickened. This process incorporates air and helps dissolve the sugar.

Gradually whisk in 500ml (2 cups) of whole milk into the egg yolk mixture until thoroughly combined. Ensure there are no lumps of sugar.

Pour the mixture into a medium saucepan. Cook over medium-low heat, stirring constantly with a wooden spoon or heatproof spatula. It's crucial to stir constantly to prevent the eggs from scrambling.

Continue cooking and stirring until the mixture thickens enough to coat the back of a spoon. The ideal temperature is around 71°C (160°F). Do not let it boil, as this will cause the eggs to curdle.

Remove the saucepan from the heat immediately. Stir in 250ml (1 cup) of heavy cream, 1/2 tsp of ground nutmeg, 1 tsp of vanilla extract, and 1/4 tsp of salt. Mix until everything is well incorporated.

Transfer the eggnog mixture to a clean bowl. Cover it with plastic wrap, pressing the wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 2-4 hours, or preferably overnight, to allow the flavors to meld and the mixture to chill thoroughly.

Just before serving, in a separate clean bowl, beat the 6 egg whites with an electric mixer on medium speed until soft peaks form. Gradually add about 2 tablespoons of sugar (from the original 1 cup if reserving some, or just a small amount) and continue beating until stiff, glossy peaks form. This adds a lighter texture.

Gently fold the whipped egg whites into the chilled eggnog mixture. Be careful not to overmix, as you want to maintain the airy texture.

If making an alcoholic version, gently stir in your chosen spirits (120-240ml / 1/2 - 1 cup of bourbon, brandy, or rum) at this stage. Taste and adjust seasoning if necessary.

Serve the eggnog chilled in individual glasses or mugs. Garnish with a sprinkle of fresh nutmeg.