Combine the espresso and sugar, stirring until the sugar dissolves. Stir in the liqueur.
Chill the mixture in the refrigerator for 1 hour, then transfer it to a metal bowl and place it in the freezer.
Every 30 minutes, whip the mixture, using a manual or electric beater.
Continue freezing and beating the mixture until it is uniformly slushy, about 2 hours.
Freeze another hour before serving.
Spoon the granita into goblets and, if desired, serve topped with whipped cream.