For the syrup, bring the sugar and water to a boil in a small saucepan. Cool and stir in the rum.
For the chocolate filling, cut the chocolate finely and set aside. Bring the cream to a boil in a saucepan and remove from the heat. Stir in the chocolate and allow to stand about 2 minutes so that the chocolate melts. Whisk until smooth. Whisk in the rum, scrape into a bowl, and cool at room temperature or in the refrigerator until set and thickened.
For the zabaione, place the egg yolks in a medium bowl and whisk in the sugar in a stream. Sift the flour over the mixture and whisk it in smoothly. Whisk in the Marsala and optional rum.
Pour the mixture into a nonreactive saucepan, preferably enameled iron, and set the pan over medium heat. Whisk constantly until the zabaione thickens and comes to a boil. Allow to boil, still whisking, for 1 minute.
Scrape the zabaione into a clean bowl, press plastic wrap against the surface, and refrigerate it to cool.
To assemble the cake, trim the top of the torta margherita so it is even and slice into 4 layers. Place one layer on a cardboard disk and moisten with one-fourth of the syrup. Spread the layer with half the cooled zabaione. Top with another layer and moisten with another fourth of the syrup. Beat the chocolate filling, either with a hand mixer set at medium speed or in a heavy-duty mixer fitted with the paddle, for about 1 minute, until it has lightened. Spread half the filling on the layer. Top with another layer and moisten with another fourth of the syrup. Spread the layer with the remaining zabaione. Top with the last layer and moisten with the remaining syrup. Use half the remaining chocolate filling to cover the top and sides of the cake. Cover the entire outside of the cake with the chocolate shavings, pressing them against the cake with a metal cake spatula. Use the remaining filling to pipe a border around the top of the cake, using a pastry bag fitted with a ¼-inch star tube (Ateco #1). Chill the cake to allow the fillings to ret.
Serve the cake at room temperature. Store leftovers in the refrigerator.
Chocolate shavings for finishing
© 1990 Nick Malgieri. All rights reserved.