How to Make It
Line a 2-quart loaf pan with plastic wrap. In a medium stainless steel bowl, whisk the egg yolks with 1/3 cup of the sugar. Set the bowl over a medium saucepan filled with 1 inch of barely simmering water, and using a large whisk, beat constantly until the mixture is pale and fluffy and doubled in volume, about 5 minutes; do not let the eggs get too hot or they will scramble. Add the Marsala and Cognac and whisk constantly until thickened, very warm to the touch and triple the original volume, about 5 minutes longer. Remove the bowl from the saucepan and let the zabaglione cool to room temperature.
In a large stainless steel bowl, beat the heavy cream until soft peaks form. Fold in the zabaglione. In another large stainless steel bowl, beat the egg whites until foamy. Add the cream of tartar; beat until soft peaks form. Add the remaining 1/2 cup of sugar very gradually; beat until the egg whites are very stiff and shiny.
Using a large rubber spatula, fold one-third of the beaten whites into the zabaglione cream. Gently fold in the remaining beaten whites in 2 additions, until only a few streaks of white remain. Spoon one-third of the semifreddo mixture into each of 2 bowls. Fold the dissolved espresso into the semifreddo mixture in one bowl and the grated chocolate into that in the other. Cover and refrigerate both semifreddo mixtures.
Spoon the remaining plain semifreddo mixture into the prepared loaf pan and smooth the surface. Freeze until firm, about 30 minutes. Spread the chocolate semifreddo mixture over the plain semifreddo and smooth the surface. Freeze until firm, about 30 minutes. Repeat with the espresso semifreddo mixture and freeze for 1 hour, then cover with plastic and freeze for at least 6 hours or for up 24 hours.
To unmold, run the sides of the loaf pan under hot water for a few seconds. Invert onto a platter. Thickly slice the semifreddo with a warm serrated knife, rinsing the knife in warm water and drying it between cuts. Set the slices on chilled plates and serve with the Espresso Granita with Whipped Cream.