Add eggnog to a small saucepan on the stove. Heat over medium. Do not boil. We just want to heat until small bubbles and steam form.
While the eggnog heats, make some espresso. You can also substitute strong coffee or make espresso using the instant espresso powder if you don't have a machine.
When the eggnog begins to steam and small bubbles form on the surface, turn off the heat. Grab a whisk and beat until frothy. Pour over the espresso mixture.