This recipe may be doubled and made two days in advance. Add the Kahlua and 1 cup whipped cream just before serving.
Fill a large bowl with ice and water, and set aside. Place 2 cups milk in a small saucepan set over medium-high heat, and bring to a boil. Place the egg yolks and sugar in the bowl of an electric mixer fitted with the whisk attachment, and whip until pale and thick. Add the hot milk, whisking constantly, in a slow stream. Return the mixture to the pot, and cook, stirring constantly, over medium-low heat, until thick enough to coat the back of a wooden spoon. Immediately add 1 cup cream and the remaining 1 1/2 cups milk, and transfer mixture to the ice bath.
When ready to serve, add the vodka and Kahlua. Whip the remaining cup cream to soft peaks. Divide the eggnog among six cups, and garnish with whipped cream. Serve.