Stir together eggs, sugar, and salt in heavy dutch oven.
Gradually add milk.
Cook over low heat 15-20 minutes, stirring constantly, just until mixture coats spoon.
Remove from heat.
Stir in vanilla.
Stir together coffee and liqueur.
Gradually stir coffee mixture into egg mixture; keep warm.
Just before serving, beat whipping cream and brown sugar in chilled large bowl with electric mixer on high until stiff.
Gently stir 2 cups of whipping cream into egg mixture.
Pour into mugs.
Top with dollops of remaining whipped cream.
Sprinkle with cinnamon.
Serve immediately.
Immediately refrigerate any remaining nog.
