Bring cream and half-and-half to simmer in heavy large saucepan. Add sugar and cocoa and stir until sugar dissolves. Remove from heat. Add chocolate and stir until smooth. Gradually whisk 1/2 cup chocolate mixture into eggs. Return to saucepan. Stir over medium-low heat until mixture thickens and leaves path on back of spoon when finger is drawn across, 10 to 15 minutes.
Strain into bowl set over larger bowl filled with ice. Cool completely, stirring often.
Stir whiskey into custard. Transfer custard to ice cream maker and freeze according to manufacturer's instructions. Freeze in covered container several hours to mellow flavors. If frozen solid, allow to soften before serving.
Makes: 4