Separate the eggs and place the yolks in a bowl. The whites can be put into an airtight container and refrigerated for later use as we only need the egg yolks for this recipe.
Slowly pour the milk into the egg mixture in a single, steady stream while whisking continuously. Make sure to add the milk mixture in a thin, steady stream so that you don't scramble the eggs.
Transfer the mixture back to the saucepan and cook on medium heat till the eggnog thickens to your desired consistency, about 4-5 minutes. Keep stirring using a whisk or a rubber spatula, as it heats to keep it from sticking to the bottom.