First, prepare a straight-sided soufflé dish. In order that the soufflé rise 1-1/2 inches over the top, the dish should not have more than a 5-cup capacity. (My glass one measures 6 inches in diameter and 3 inches in depth. The closest white china soufflé dish measures 6-1/2 (across the top) x 3 inches-and it works fine.) Prepare an aluminum foil collar: Tear off a piece of foil large enough to wrap around the dish and overlap a few inches. Fold it in half the long way. With a paper towel, brush tasteless salad oil over half of one of the long sides, brushing it along the half that has two open sides, not the folded edge. Wrap the foil tightly around the dish, oiled side to the top and inside. Fasten tightly with a string. Set aside.

Place the cold water in a small bowl or a cup with at least 1-cup capacity. Sprinkle the gelatin over the top and let stand for 5 minutes. Then dissolve the coffee in the boiling water, quickly add it to the gelatin and stir to dissolve. Set aside.

Place the milk and both chocolates in the top of a small double boiler over hot water on moderate heat. Stir occasionally with a small wire whisk until the chocolate is melted and the mixture is smooth. Remove the top of the double boiler and set it aside.

In the small bowl of an electric mixer, beat the egg yolks with 1/2 cup of the sugar (reserve remaining 1/2 cup). Beat for a few minutes at high speed until the mixture is creamy and pale-colored. Beat in the vanilla and then, on low speed, gradually add the warm chocolate mixture, scraping the bowl with a rubber spatula and beating until smooth. Gradually beat in the dissolved gelatin. Transfer to a medium-size mixing bowl (preferably metal) and set aside.

Prepare a large mixing bowl partly filled with ice and cold water and have it ready.

In the large bowl of the electric mixer (with clean beaters) beat the whites and the salt until the mixture increases in volume and starts to thicken. Gradually, while beating on moderate speed, add the reserved 1/2 cup sugar. Then increase the speed to high and beat only until the mixture holds a soft peak-one that bends over slightly when the mixture is lifted with a rubber spatula. (If the whites are beaten until stiff or dry, it will be impossible to fold the chocolate into them without losing most of the air that has been beaten in.) Remove from the mixer and set aside.

Place the bowl of chocolate mixture in the ice and water and stir frequently until the mixture thickens to the consistency of a medium cream sauce. (This is an important step; if the chocolate mixture is too thin when it is folded into the whites, the chocolate will sink to the bottom-if it is too thick, it will become lumpy and the mixture will not be smooth. So pay close attention to it. Stir constantly after it starts to thicken slightly. It might take about 10 minutes, or a bit more. Actually, the chocolate mixture and the beaten whites should be of the same consistency-or as close as possible-for easy folding.)

Just as soon as the chocolate is read, remove it from the ice water and fold a few large spoonfuls into the beaten whites. Repeat two or three times, folding in about three-fourths of the chocolate. Then fold the whites into the remaining chocolate. If necessary, pour gently back and forth from one bowl to another to insure thorough blending.

Gently pour the soufflé into the prepared dish and place it in the refrigerator. To keep the air out, place a piece of plastic wrap over the top, letting it rest on the foil collar, not touching the soufflé. Let stand for about 8 to 10 hours or overnight.

OPTIONAL: Finely grate 1 oz of semisweet chocolate and sprinkle it over the soufflé before removing the collar.

Do not remove the collar until shortly before serving. Peel it off very gently or, if it sticks, cut between the soufflé and the collar with a small, sharp knife. Wipe the sides of the soufflé dish and place it on a folded napkin on a flat plate.

Serve with a side dish of whipped cream.

Beat the above ingredients to make soft whipped cream and serve a large spoonful over each portion. (If the cream is whipped ahead of time it may separate slightly; if so, just stir it a bit to blend before serving.)

NOTE: The effect will not be so dramatic, but this may also be prepared in any serving bowl or in individual wine glasses or dessert bowls.