To make the cake:

Preheat the oven to 325°. Butter an 8- or 9-inch cake pan and line the bottom with parchment paper. Butter the parchment (this allows it to peel more easily from the cake bottom).

Sift together the flour, baking soda, and salt, and set aside. In a small bowl, whisk together the cocoa and boiling water until smooth. Let cool to room temperature.

Combine the vanilla, mayonnaise, and sugar in a bowl and beat with an electric mixer until light. Start beating on slow speed and increase to medium until the mixture is light and all the sugar has been incorporated. Scrape down the sides of the bowl. Reduce the speed to low and add the cocoa mixture in a slow, steady stream. Add the flour mixture and fold in while turning the bowl to scrape the sides and bottom. Mix just until the flour disappears. You should have a smooth, velvety batter.

Scrape the batter into the prepared pan. With a small, offset spatula, smooth the top. Bake until puffed and a cake tester inserted into the center comes out clean, or the top springs back when lightly pressed with a finger, about 50 minutes. Remove from the oven and let cool completely in the pan. Run a knife around the edge to loosen the sides and invert the cake onto a plate. Peel off the parchment. You will have one 8- or 9-inch cake that will serve 6 on its own. (The cake may be made a day ahead. Wrap well.)

To make the mousse:

Combine the chocolate and cream in a heatproof bowl and place over (but not touching) barely simmering water. Heat, stirring occasionally, until melted, smooth, and shiny. Remove from the heat.

In a large mixing bowl, beat the cream cheese and mascarpone with an electric mixer at medium to medium-high speed until soft and creamy. Pause occasionally to scrape down the sides of the bowl. On low speed, beat in the chocolate mixture.

In another bowl, with clean beaters, whip the egg yolks until frothy. Add the sugar and salt and continue to beat on high speed until the mixture is pale yellow and forms a ribbon when the beaters are lifted.

With a rubber spatula or wooden spoon, mix together the yolks and cheese mixture until smooth. Cover and refrigerate until ready to assemble the tiramisù. You should have about 2 cups mousse. (Served on its own, the mousse will serve 4. The mousse may be made ahead, covered, and refrigerated for up to 1 day.)

To make the zabaglione:

Prepare an ice waterbath. Combine the yolks, rum, and Marsala in the top of a double boiler. Place over (not touching) barely simmering water in the lower pan and whisk vigorously, or beat with an electric mixer, until the mixture begins to foam. Add the 1/2 cup sugar and whisk continuously until the mixture triples in volume and thickens to form soft mounds when the whisk is lifted, about 7 minutes. Immediately place the top of the double boiler in the ice water bath to stop the cooking. Let cool, stirring occasionally.

In a small bowl, beat the cream until it begins to foam. Add the remaining 2 teaspoons sugar and continue to beat until the cream holds soft peaks. Fold into the zabaglione. Cover and chill for several hours before assembling the tiramisù. You should have about 2 cups, which will serve 4 on its own.

To assemble:

In a small bowl or pitcher, add the sugar to the hot espresso. Stir well to dissolve the sugar, then add the rum. Reserve.

Cut off the top of the cake and reserve for another use (see Chef's Note). Cut the cake in half horizontally. Arrange a layer of cake, cut side up, in the bottom of a deep 2-1/2-quart bowl. Moisten evenly with some of the espresso syrup. Spread half the mousse on top, and then spread half of the zabaglione on top of the mousse. Arrange the second layer of cake, again cut side up, on the mousse. Moisten well with syrup. Spread the remaining mousse on top in an even layer, followed by the remaining zabaglione. Cover and refrigerate for several hours or overnight before serving. With a vegetable peeler, cut curls of chocolate to garnish the top, or dust with cocoa.

If you use an 8-inch pan for the cake, it will rise over the edge by about 1/4 inch. If you are planning to use the cake for birthday cake, you will have to trim the cake to get an even edge. If you are concerned about using raw eggs in the mousse, use pasteurized eggs or a pasteurized egg substitute, using the equivalent of 2 whole eggs. The result will be lighter and delicious.

The shiny top of the cake will not readily absorb syrup. If you do not cut it off, you might find dry spots in your tiramisù. But don't throw the top out! Crumble it into crumbs and place them in a 150°F to 200°F oven until thoroughly dry and crunchy. Use to decorate the top of the tiramisù, or save them to add to cookie batters or to scatter in pies and tarts to absorb juices.