Makes 6 servings

Preparing the cake: Position rack in center of oven and preheat to 350°. Grease a 17 1/2" x 12 1/2" jelly roll pan with vegetable oil spray, and line with parchment or waxed paper. Spray the paper lightly but completely with oil.

Sift together 1 cup of the sugar and the other dry ingredients onto a sheet of waxed paper. In a large bowl, whisk together all wet ingredients except for the egg whites until blended. Add dry mixture slowly, whisking all the while. In a separate bowl, whip egg whites at a low speed with an electric mixer until they are foamy and form soft peaks. Increase mixer speed and gradually add remaining 1/2 cup sugar until whites are thick and shiny and hold peaks. Fold 1/3 of the whipped egg whites into the yolk mixture to lighten it, then turn the yolk mixture into the whites and fold it gently but thoroughly.

Pour and scrape the batter into the prepared pan, spreading it evenly with an offset spatula. Bake the cake for 10 to 12 minutes, until the edges start to pull away from the sides of the pan. Transfer the pan to a rack and allow to cool to room temperature.

Preparing the mousse: Process the walnuts and walnut oil in a food processor, fitted with the metal blade, until the mixture forms a paste (scrape sides as necessary). Add melted chocolate and process until blended. Set aside. Whisk egg yolks lightly by hand in a heavy duty mixer bowl. Bring 2 inches of water to a boil in a saucepan to be used as a double boiler with the mixer bowl, reduce heat and keep at a simmer while you proceed. In a small saucepan, combine sugar and water and bring to a boil. Remove pan from heat, and very slowly add the boiling syrup to the egg yolks, whisking mixture constantly. Fit the bowl to the saucepan of simmering water and continue to whisk mixture until it is foamy and hot to the touch. Remove bowl from heat and fit it into the mixer. With the whisk attachment, beat mixture on high until it is pale, holds a soft ribbon and is cool to the touch. With a rubber spatula, fold in walnut paste. Whip the heavy cream until it holds soft peaks. Fold lightly but thoroughly into walnut mixture. Refrigerate, tightly covered, until needed. Stir before using.

To finish the cake: Lightly sprinkle top of cake with confectioner's sugar and cover with a piece of parchment or waxed paper. Place an inverted jelly roll pan over the cake and turn the cake over. Remove the baking pan, peel off the paper, turn paper over and reposition it over the cake, Invert the baking pan, press against the cake, and invert it again so that the cake is right side up and resting on the inverted pan. Trim edges of the cake with a long thin knife. Keeping the 17" side of the cake in front of you, spread the chilled mousse over the cake, leaving 1 inch of the cake bare along the side closest to you, roll the cake up into an even roll, using the paper to help you move it along. When you come to the edge of the roll, the edges of the paper should meet.. Grab the edges and use the paper to help you scoot the cake into the center of the pan. Use a fork to tuck edges of paper under the cake and refrigerate for 2 hours (overnight if wrapped well).

To serve: Cut 5 strips of waxed paper and lay across the cake at evenly spaced intervals. Dust the cake with cocoa powder and a hint of confectioner's sugar. Lift off the paper. Place 1 caramelized walnut (optional) on each undusted stripe. Using 2 spatulas, lift cake onto serving platter and garnish with sprigs of fresh mint (optional).