To make the soufflé mixture:

Separate the eggs, putting the yolks in a mixing bowl and the whiles in the bowl in which they will be beaten. Beat the yolks until light and lemon-colored, then gradually beat in the sugar and salt until the mixture is very thick, pale yellow, and ribbony.

Meanwhile, melt the chocolate in the top of a double boiler over hot water or in a small pan in a 300°F oven.

Stir the chocolate and vanilla into the egg-and-sugar mixture and beat until well mixed. Preheat the oven to 375°F.

Beat the egg whites until foamy; add the cream of tartar (unless you are using a copper bowl) and continue to beat until they hold soft, unwavering peaks. Fold into the chocolate mixture rather more completely than as for other soufflés—it is not essential to have bits of unincorporated egg white, as this version of the soufflé is meant to fall.

To make the whipped cream:

Put the chilled cream in a chilled bowl and beat with a rotary or electric beater until it has doubled in volume and holds its shape when the beater is lifted. Gently fold in the sugar and cognac, or other flavoring.

To assemble the chocolate roll:

Butter a 15 x 11-inch jelly-roll pan, then line the pan with waxed paper, leaving an overhang of about 2 inches at each end. Butter the waxed paper, to prevent the cake from sticking. Or, if you have kitchen parchment, line the pan with parchment, which does not need to be buttered. Preheat the oven to 350°F.

Spread the mixture evenly in the prepared pan, smoothing it with a rubber spatula so it is the same overall thickness. Bake on the center shelf of the preheated oven for 15 minutes, or until the top feels firm and springy when touched.

Have ready a large sheet of waxed paper or aluminum foil sprinkled with cocoa or sifted confectioners’ sugar. Remove the jelly-roll pan from the oven and, holding it carefully with pot holders, very quickly invert the pan onto the paper or foil so the cake falls onto it, top side down. Carefully peel the paper or parchment from the cake, loosening it, if necessary, with a small, sharp knife. If some of the surface sticks, this is not too important as the filling will go on this side.

Let the cake cool, and then spread it with the whipped cream, not quite to the edges. Lift one long edge of the waxed paper or foil and raise it so the cake rolls inward. Then, with the aid of the waxed paper, roll the cake onto a long narrow chocolate-roll board, or a flat serving tray. To serve, cut in diagonal slices.

Variation: Nut Roll. Substitute 1 cup finely ground nuts (pecans, walnuts, filberts) for the chocolate, folding them into the egg-and-sugar mixture before folding in the egg whites.