In a large, heavy saucepan, melt chocolate and butter over low heat, stirring
often.
Stir in sugar, corn syrup and 2/3 cup of the Half-and-Half. Stir over medium-low
heat until mixture comes to a boil. Simmer for 4 minutes without stirring; set aside.
In a small bowl, beat eggs until blended.
Stir in 1/2 cup of hot chocolate mixture.
Stir egg mixture into remaining chocolate mixture. Cook and stir over medium heat
until slightly thickened, about 1 minute.
Cool to lukewarm.
Stir in vanilla extract, whipping cream and remaining Half-and-Half.
Freeze in ice cream maker according to manufacturer's directions.