Julia Child’s Chocolate Mousse Cake

Place oven rack in lower third of oven and preheat oven to 350°F. Butter a (preferably nonstick) 9-by-9-by-2-inch cake pan. Cut a square of parchment or waxed paper to fit bottom of pan; butter parchment and place in pan. Dust inside of pan with flour; tap out any excess. Choose a roasting pan large enough to hold the cake pan easily, fill with enough hot water to come halfway up the side of the cake pan and set in the oven.

Swirl coffee and hot water in a medium saucepan, add rum and add chocolate to pan. Bring 2 inches of water to a boil in a larger pan, remove from the heat and set the chocolate pan in it; cover and let the chocolate melt while you continue with the recipe.

In a mixer bowl, stir eggs and sugar over hot water for several minutes until eggs are slightly warm to your finger. Then beat for 5 minutes or more until te mixture has at least tripled in volume and forms a thick ribbon when a bit is lifted and falls from a beater; egg should be the consistency of lightly whipped cream.

In a metal mixing bowl set in a larger bowl of ice and water, beat cream with a hand-held mixer or a large balloon whisk, using an up-and-down circular motion slowly whip in as much air as possible, until cream has doubled in volume and holds its shape softly. Whip in vanilla.

Whisk melted chocolate mixture until smooth and silky. Scrape it into egg-sugar mixture, blending rapidly with a rubber spatula. When chocolate is partially incorporated, fold in whipped cream, deflating cream and egg a little as possible.

Turn batter into prepared cake pan; it will be about two-thirds filled. Set it at once in pan of hot water in preheated oven. Bake until cake rises about 1/8 inch above edge of the pan and a skewer inserted in the cake comes out clean, about 1 hour. Turn off oven, leave door ajar and let cake sit for 30 minutes in its pan of water; cake will sink evenly this way.

Remove cake in its water bath from oven and let it for another 30 minutes, so that it will firm up before unmolding and serving. The cake will sink down as it cools to about its original volume.

Unmold cake onto a serving platter and decorate with a sprinkling of confectioners' sugar or with pipings of whipped cream. Pass sweetened, vanilla-flavored whipped cream or a custard sauce with the cake.