Eggnog can be prepared and enjoyed hot or cold, but regardless of your temperature preference for this classic drink, the consistency should be thick and creamy. A failure to cook eggnog long enough or beat the egg yolks or whites long enough or with the appropriate tool can result in eggnog that lacks its signature body. Prevent thin eggnog by cooking it until it reaches the desired consistency, or remedy a runny batch with the addition of beaten egg whites or heavy cream.
Eggnog prepared on the stovetop should be gently cooked until it reaches a temperature in the 150 to 160 degree Fahrenheit range. Once you see steam rise, whisk vigorously for an additional 30 seconds to a minute, until the eggnog becomes thick and smooth. You want the eggnog to coat the back of a spoon; don't pull the eggnog from the heat until it reaches this consistency. You can serve it hot, but if it's too thin, whirl it in a regular blender for 20 to 30 seconds to add more body. Or, if you prefer to serve it cold, let the eggnog chill in the refrigerator; it will thicken over the course of a few hours.
While the cooking process thickens hot preparations, cold eggnog relies on egg yolks and well-beaten egg whites for the desired consistency. First, beat the egg yolks with an electric mixer, not a whisk. Then beat the egg whites in a separate bowl until stiff peaks form, which takes approximately 3 to 5 minutes. To give thin eggnog more body, try adding another well-beaten egg white or two, or beat heavy cream until it's thick, approximately 30 to 60 seconds, and gradually add a small amount until you reach the desired consistency.