1 1/2 cups sugar
12 large eggs, lightly beaten
4 cups half-and-half
4 cups milk
1/4 teaspoon salt
4 1/2 cups chilled strongly brewed coffee
3 cups Irish cream liqueur
2 teaspoons vanilla extract
2 cups whipping cream
Garnishes: additional whipped cream, thin chocolate mints
1. Gradually add sugar to eggs in a large glass mixing bowl, whisking until blended. Set aside.
2. Stir together half-and-half, milk, and salt in a Dutch oven over medium-low heat. Cook, stirring occasionally, 12 to 15 minutes or just until mixture begins to bubble around edges of pan. (Do not boil.)
3. Gradually stir half of hot milk mixture into egg mixture. Stir egg mixture into remaining hot milk mixture in Dutch oven.
4. Cook mixture over medium-low heat, stirring constantly, until mixture slightly thickens and a thermometer registers 160° (about 25 to 30 minutes). Remove from heat, and stir 1 minute. Pour mixture through a fine wire-mesh strainer into a serving bowl. Stir in coffee, Irish cream liqueur, and vanilla; let cool 1 hour. Cover and chill at least 4 hours.
5. Beat whipping cream at high speed with an electric mixer until soft peaks form. Fold whipped cream into chilled eggnog, and garnish, if desired.