The chocolate should be 70% cacao chocolate such as Lindt, finely chopped.
Bring milk, cream and vanilla to a boil (just). Remove from heat, cover, and keep warm.
Put 1/2 cup sugar and 3 TBLS water in a pan and bring to a boil. Keep stirring until sugar is disolved and wash down any sugar crystals on the wide of pan with a pastry brush dipped in cold water. Boil syrup, without stirring, gently swirling pan, and washing down crystals, until mixture is a deep golden carmel, about 10 minutes. Remove pan from heat and carefully whisk in warm cream mixture (mixture will steav vigorously and caramel may harden). Cook over low heat, whisking, until caramel is dissolved.
Beat yolks with salt and remaining 1/4 cup sugar, using mixer at high speed, until trippled in volume and able to form a ribbon that takes 2 seconds to dissolve into mixture when beater is lifted (6 - 8 minutes with handheld mixer).
Slowly add caramel mixture to yolks in a slow stream, whisking. Transfer to 3 qt. saucepan. Cook over low heat, stirring constantly, until custard is slightly thickened and registers 170 on thermometer.
Force custard through sieve into bowl with the chopped chocolate, and let stand 1 minute, the whisk mixture until smooth.
Cool to room temperature, stirring occassionally, for about 35 minutes.