Into a saucepan, sift together the cocoa and the sugar
and stir in the heavy cream and the half and half.
Add the chocolate, heat the mixture over moderately low heat,
stirring, until the chocolate is melted, and remove the pan from the heat.
In a bowl, whisk the eggs until they are combined well;
add about one fourth of the chocolate mixture, whisking,
and whisk the egg mixture into the remaining chocolate mixture.
Cook the mixture over moderate heat, stirring, for 4-5 min.,
or until it coats the back of the spoon, but do not let it boil.
Transfer the mixture to a metal bowl set in a larger bowl of ice
and cold water and chill it, stirring, until it is cold.
In a small saucepan, heat the Jack Daniels over moderate heat
until it is warm and ignite it carefully, shaking the pan gently
until the flames go out. Stir the Jack Daniels into the chocolate mixture
and freeze the mixture in an ice cream freezer according to
the manufacturer's instructions. Makes about 3 quarts.