To make the genoise, preheat the oven to 375 degrees. Grease and lightly flour a 10” cake pan with sides three inches high. Sift together the flour, cocoa, baking powder, and baking soda. In a medium bowl set over a pan of simmering water, beat the eggs and sugar together until the mixture has warmed and most of the sugar has dissolved, about three minutes.
Using a hand-held electric mixer, beat the egg-sugar mixture at a high speed until doubled in volume, about five minutes. Off heat, fold in the sifted dry ingredients, pour the batter into the prepared cake pan, and bake about 25 minutes, until a tester inserted into the center of the cake comes out clean and the cake’s surface springs back when lightly pressed. Cool completely for two to three hours.
While that’s cooling make the syrup. Warm the Marsala in a saucepan and whisk in the sugar until dissolved.
To make the filling, soften the gelatin in 1 ½ tablespoons of cold water. In a medium-size heavy saucepan, combine the sugar with the ¼ cup of water and bring to a boil over moderately high heat. Cook until the resultant syrup registers 250 degrees on a candy thermometer, or forms a ball when dropped into cold water, about 5 minutes.
Meanwhile, beat the egg yolks in a stainless-steel bowl. Set the bowl over a pan of lightly simmering water and beat until warmed through but not cooked. Beat in the sugar syrup until well blended, and remove from the heat.
Melt the softened gelatin in the water in a small saucepan over low heat until completely dissolved. In a small bowl set over a pan of gently simmering water, melt the chocolate until smooth but not hot. Beat the chocolate into the egg-sugar mixture, and then beat in the dissolved gelatin and the Marsala. Fold in the whipped cream until no traces of white are visible.
To assemble the cake, use a long, serrated cake knife, to slice the chocolate sponge cake horizontally to form three layers of equal thickness. Set the bottom layer into 10 inches spring-form pan and brush the surface with Marsala syrup, moistening it well/ Spoon 1/3 of the filling over the surface, and repeat all steps with the remaining two layers of sponge cake, ending with filling to cover the surface of the top layer. Smooth with a spatula, cover, and refrigerate overnight.
The sauce should be started by bringing three inches of water to a simmer. In a large cooper or stainless-steel bowl, whisk together the egg yolks, Marsala, and sugar. Set the bowl over the simmering water and whisk constantly until the mixture is fluffy, light, and hot, three to four minutes. Off the heat, very slowly and thoroughly fold in the melted chocolate.
Remove the cake from the spring-form pan and sift the unsweetened cocoa over the top. Serve in slices with hot Chocolate Zabaglione Sauce.