Scald the milk in a medium saucepan. In a medium heatproof bowl, whisk the egg yolks with the eggs, sugar and salt. Slowly whisk in half of the hot milk, then whisk in the remaining milk. Return the sauce to the saucepan and cook over low heat, whisking constantly, until the custard is thick enough to coat the back of a wooden spoon, about 6 minutes.
Strain the custard through a fine sieve and let cool completely. Stir in the Maraschino liqueur and vanilla and refrigerate until chilled.
The custard can be refrigerated for up to 2 days. Bring to room temperature before serving.