Grease a 15 1/2 x 10 1/2 x 1 inch jelly roll pan. Line the bottom and sides with a sheet of foil; grease the foil. Preheat oven to 350°, setting rack in center.
Melt the chocolate with the butter in a small, heavy saucepan over low heat, stirring occasionally. When the mixture is melted and smooth, set aside to cool. In a large bowl, using a whisk or electric beaters, beat the eggs with the sugar until light and creamy. Add the chocolate mixture, and continue to beat just until well blended. Sift together the flour, baking powder and salt. Stir flour mixture into the egg mixture, blending well, but do not beat. Add the vanilla and walnuts.
Pour the mixture into the prepared pan; shake it gently from side to side to level, making certain that the corners are not slighted. Bake for 18-20 minutes, reversing the pan from front to back, if necessary (toward the end of cooking time), to ensure even baking. The cake is done when it seems firm to a light touch and a pick comes out just barely dry. Do not overbake. Set the pan on a wire rack to cool completely. To remove, place a large cookie sheet over the pan; invert and lift off the pan; carefully peel away the foil. Note: The torte should be put together soon after cooling.
To assemble, first prepare the whipped cream filling and frosting (recipe follows). Trim any uneven edges, and cut the cake across the length into four sections, each about 3 3/4 inches wide and 10 inches long. Place one section on a large platter and spread with a layer of whipped cream; repeat with the remaining layers. Frost the top and sides with the remaining whipped cream. (Count on about half the mixture for the filling, the remainder for the frosting.) Sprinkle the top of the torte with the chocolate shavings.(*Shavings are quickly made by holding a block of semisweet chocolate in your hand to warm it slightly; shave as many strips as desired with a vegetable peeler.) Refrigerate at least 24 hours before serving. Remove the torte from the refrigerator 30 minutes before serving. Cut into thick slices. 12 servings.
In a chilled bowl, with chilled beaters, whip the cream with the sugar until very stiff, but not buttery. Refrigerate briefly while preparing the torte layers.