Oil the bottom of a 15 1/z x lO 1/2h x 1-inch jelly roll pan. Cut a piece of waxed paper to fit the width of pan and extend about 1 inch over ends. Place the chocolates and the water in the top of a double boiler over hot water. Stir with a wooden spoon until the chocolate is melted, then cool. Place the egg yolks in a large mixer bowl and beat slightly with the mixer, Add the sugar gradually, beating constantly until all the sugar is dissolved and the mixture is thick and of a batter consistency. Stir in the chocolate with a rubber spatula until well combined and free of streaks. Beat the egg whites until stiff peaks form. Fold about 1/3 of the egg whites into the chocolate mixture thoroughly to lighten, then fold in the remaining egg whites carefully but completely with a rubber spatula. Spread the batter evenly in the prepared pan. Bake in a preheated 350 degree oven for 18 minutes. Remove from oven and cover top with damp towels. Let stand for 30 minutes or until cooled. Remove the towels carefully, then loosen by lifting the extending ends of waxed paper. Place cocoa in a fine sieve and dust top generously. Place a piece of waxed paper over the top, then place a cookie sheet on top and invert. Peel the waxed paper carefully from the cake. Place the cream in a large mixer bowl and beat with an electric mixer until soft peaks form. Sprinkle in the confectioners' sugar and beat until stiff. Stir in the vanilla. Spread the whipped cream evenly over the cake. Roll the cake from the long side, as for a jelly roll, directly onto a serving tray. Roll with care as this cake is extremely tender. Dust the top generously with cocoa. You may refrigerate the roll for several hours before serving. Cut into slices to serve.