In a heavy medium saucepan, combine the chocolate and half-and-half. Cook over
moderate heat, whisking occasionally, until the chocolate is melted and the mixture
is smooth, about 5 minutes.
In a small bowl, combine the sugar, flour and salt.
Gradually whisk the dry ingredients
into the melted chocolate and bring to a boil. Reduce the heat to moderately low
and cook, whisking occasionally for 1 minute.
In a medium bowl, whisk the eggs briefly.
Slowly whisk in about half of the chocolate
mixture, then transfer this mixture to the remaining chocolate in the saucepan.
Cook the custard until it's thick, smooth and steaming and reaches about 160
degrees F on a candy thermometer.
Remove from the heat and stir in the heavy cream and vanilla extract.
to room temperature, then refrigerate or chill in a water bath until cold.
Pour the custard into an ice cream maker and freeze according to the manufacturer'
When frozen, transfer the ice cream to a container, cover and freeze for at least
30 minutes before serving. (The ice cream can be made up to 2 days before serving.)