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