Grease bottom and sides of a 15- x 10-inch jellyroll pan; line with wax paper. Grease wax paper and dust with cocoa. Set prepared pan aside.
Beat egg yolks at high speed with an electric mixer until foamy. Gradually add 1/4 cup granulated sugar, beating until thick and pale. Gradually stir in 1/2 cup cocoa, 1 1/2 teaspoons vanilla, and pinch of salt, beating until well blended.
Beat egg whites until soft peaks form; gradually add remaining 1/2 cup granulated sugar, beating until stiff peaks form. Fold gently into cocoa mixture. Spread batter evenly into prepared jellyroll pan.
Bake at 375° for 12 to 15 minutes or until a wooden pick inserted in center comes out clean.
Sift 2 tablespoons cocoa in a 15- x 10-inch rectangle onto a cloth towel. Remove cake from oven, run a knife around edges of pan to loosen cake, and turn cake out onto prepared towel.
Peel wax paper off cake; trim cake edges, and discard. Starting at a short end, roll up cake and towel together; place, seam side down, on a wire rack to cool.
Beat whipping cream at low speed with electric mixer until foamy; add 1/2 cup powdered sugar, remaining 1/4 cup cocoa and remaining 1 teaspoon vanilla. Beat at high speed until soft peaks form.
Unroll cake, and spread evenly with whipped cream mixture, leaving a 1-inch border around edges. Reroll cake without towel, and place, seam side down, on serving platter; garnish, if desired.