Recipe: Scald the milk in a saucepan. Put the lemon zest and vanilla in a bowl, and add the hot milk. Let it cool down, cover and refrigerate until completely cold. Strain the milk into a saucepan, add half the sugar, and bring to a simmer. Whisk the egg yolks with the remaining sugar until creamy. Gradually dribble the hot milk into the egg yolks, whisking. When all the milk has been added, cook the mixture over a double boiler, stirring continuously until the mixture is thick enough to coat the back of a spoon. Add a pinch of salt and strain into a bowl set over crushed ice. Cover, and refrigerate until cold. Transfer to an ice-cream machine and follow the instructions of the machine. Transfer the gelato to a container and return to the freezer until very stiff. Serve the gelato in chilled glasses and sprinkle a few strands of lemon zest over each one.