In a large saucepan, whisk together 6 whole eggs, 240ml sugar, and a pinch of salt until fully combined and slightly pale.
Gradually whisk in 480ml whole milk, then place the saucepan over low heat.
Cook, stirring constantly with a wooden spoon or heatproof spatula, until the mixture thickens enough to coat the back of the spoon and reaches 74°C (160°F) on an instant-read thermometer - approximately 15–20 minutes. Do not allow the mixture to boil.
Remove from heat immediately. Stir in the remaining 480ml whole milk, 5ml vanilla extract, and 180ml aged dark rum.
Transfer to a covered container and refrigerate until thoroughly chilled, at least 4 hours or overnight.
When ready to serve, pour into a chilled pitcher or punch bowl, dust generously with freshly grated nutmeg, and serve in individual glasses.
Cooking the custard base to exactly 74°C is the key to both food safety and texture - pull it too soon and it stays thin, push it past a boil and you'll scramble the eggs. Adding the second half of the milk off-heat stops the cooking instantly and gives you that silky, pourable consistency.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Egg-Nog - Classic Cooked.