Break 2 candy bars into pieces. Process candy bar pieces in a food processor until finely crushed. Place on a shallow plate, and set aside. Chop remaining 3 candy bars.
Place bittersweet chocolate in food processor bowl, and pulse until finely chopped. Combine whipping cream and butter in a 1-cup glass measure; microwave at HIGH 1 minute or until butter is melted and cream begins to boil. Pour hot cream mixture through food chute with processor running; process until mixture is smooth. Add butter and nut flavoring; process until blended. Transfer mixture to bowl; stir in chopped candy bars. Cover and chill 30 minutes.
Scoop chocolate truffle mixture into 1" balls. Roll in palms of hands to make smooth and uniform balls. Quickly roll in crushed candy bars. Store truffles in refrigerator up to one week. Let stand at room temperature before serving.