Bake a box chocolate cake as directed. While hot from the oven, poke holes over entire cake with a fork. While the cake is cooking prepare the fudge sauce.
For the fudge sauce: In a saucepan, combine butter, sugar, cocoa and cream. Stir over medium heat until a full boil starts. Cook at a full boil for 2 minutes. Stir in vanilla. Pour while hot over warm cake. Sprinkle with chopped walnuts. Serve alone or with ice-cream.