Heat oven to 350°F. Heat half-and-half in medium saucepan until very hot. Half-and half should be steaming but not bubbling. Stir in chocolate until melted. Stir in vanilla. Cool slightly. Meanwhile beat egg yolks, sugar and salt in medium bowl until blended but not foamy; slowly whisk in chocolate mixture.
Place pan on rack in center of 350°F oven; pour very hot water into pan to within 1/2 inch of top of cups. Bake until knife inserted near center comes out clean, 30 to 35 minutes; center will still be soft. Remove cups from water bath at once; cool on wire rack 5 to 10 minutes. Serve warm or refrigerate until cold.
This rich chocolate single-serving gourmet dessert can easily be made ahead for parties or holiday entertaining.
No-mess pouring: Make custard in a bowl with a pouring lip, or transfer it to a large glass measure, to make filling custard cups easier and neater.
For perfectly smooth custard: Strain it through a sieve when filling custard cups or baking dish to remove any tough egg strands.