Line a large baking sheet with foil. Put cocoa, corn syrup, whiskey and butter in a medium-size saucepan; stir over low heat until melted and well blended. Remove from heat. Stir gingersnaps and walnuts into whiskey mixture; allow to cool slightly. Roll pieces of mixture into balls about the size of a walnut, then flatten into even rounds. Place on prepared baking sheet. Refrigerate 1 to 2 hours or until firm. Sprinkle powdered sugar lightly over cookies. Remove cookies from foil and arrange on a serving plate. Store in an airtight container in a cool place for 4 to 5 days.
