Prepare the pan first; it should be an 11 x 16 x 1/2-inch pan. Butter it well, then fit wax paper in the bottom and butter the wax paper (only wax paper will work with this). Preheat the oven to 350°F.

Soften the chocolate over hot (not boiling) water and let cool to room temperature. Separate the eggs and beat the yolks until very light and lemon-colored. Add the sugar gradually, and continue beating until very light. Stir in the chocolate and beat again until the mixture is very light, adding the vanilla and salt while beating. Beat the egg whites until foamy, add the cream of tartar, and beat until the whites are light and hold a soft peak. Fold into the chocolate mixture with a rubber or plastic spatula. Turn the bowl as you do this to incorporate the two mixtures evenly and retain as much air as possible. Spread lightly into the prepared pan and bake in the preheated 350°F oven approximately 15 minutes or until the roll pulls away from the sides of the pan. Transfer to a rack, put a damp towel over the pan, and leave 10 minutes. Take off the towel, loosen the sides of the roll, and turn onto wax paper sprinkled with cocoa or powdered sugar. Cool. Fill with the whipped cream, but do not spread it Out to the edges. Lift the wax paper on one side, which will cause the cake to lift also and roll inward. Continue to roll, and roll off the paper onto a serving plate or onto a Roulage Léontine or special chocolate roll board. This roll will often crack, but it does not matter. Serve cut in diagonal slices.

This will freeze, but it is not as good. In fact I prefer it not even refrigerated. It will hold perfectly at room temperature for 4 or 5 hours.