Mix together the espresso, sugar, coffee liqueur and vanilla extract in a medium bowl until the sugar is completely dissolved. Pour the liquid into an 8-inch square glass baking dish and freeze. Every 30 minutes, remove the dish from the freezer and use a fork to scrape and break up the ice crystals. Repeat until the mixture is thoroughly frozen, about 4 hours.
Spoon the granita into small glasses, dollop with the whipped cream and garnish with a mint leaf. Serve immediately.