Mix the 1 cup sugar, salt and half-and-half in a heavy-bottom saucepan. Heat
to scalding, stirring occasionally and being careful not to boil.
Slowly whisk about a cup of the hot half-and-half mixture into the eggs, then
slowly whisk the egg mixture back into the saucepan. Cook over medium-low
heat, stirring constantly, until the mixture thickens slightly.
Remove from heat and stir in vanilla extract.
Melt the chocolate and 1/2 cup sugar in the top of a double boiler over barely
simmering water; stir until smooth.
Pour the chocolate mixture into the custard; stir until thoroughly blended.