Bring cream and half and half to a simmer in a large heavy saucepan. Add sugar and cocoa; stir until sugar dissolves. remove from heat. Add chocolate, stir until smooth. Gradually whisk half cup chocolate mixture into eggs. Return mixture to saucepan. Stir over medium-low heat for 10-15 minutes or until mixture thickness and coats the back of a spoon. Strain into bowl. Cool mixture completely, stirring often. Stir whiskey into custard. Pour into a 1 litre ice cream churn. Freeze according to manufacturer's directions. Store in a covered container for several hours to mellow the flavor. If ice cream is frozen solid, soften in refrigerator before serving.