Heat heavy cream, butter and sugar in medium saucepan over medium heat to a gentle boil. Remove from heat. Add chopped chocolate and let stand 30 seconds. Whisk until smooth. Stir in cognac.
Place saucepan in bowl of ice water. Beat chocolate mixture until it thickens, holds its shape and is firm enough to pipe, using hand-held electric mixer at medium speed.
Transfer ganache to pastry bag fitted with 1/2-inch plain tip (such as Ateco #6). Pipe 1-inch mounds onto baking sheet. Freeze truffles for 15 minutes or until just firm enough to shape.
With hands dusted with confectioners' sugar, gently roll ganache into 1-inch balls. Refrigerate about 10 minutes or just until firm.
Coat the truffles:.
Roll one-third of the truffles in confectioners' sugar. Roll another third in cocoa powder. Then roll remaining truffles in chopped walnuts. Store truffles in airtight container in refrigerator up to 5 days.
Remove truffles from refrigerator 30 minutes before serving. Place each truffle in a decorative petit four paper cup and arrange on serving plate.