In a medium bowl, whisk the egg yolks and sugar until blended. Whisk in 1/2 cup cream, the marsala and salt. Place the bowl over a large saucepan of simmering water (do not let the bottom of the bowl touch the water). Whisk constantly until the mixture is thick and a thermometer inserted into the center registers 160 to 170 degrees F, about 15 minutes. Place the bowl on a work surface and add the chocolate chips. Whisk until the chocolate chips are melted and the mixture is smooth. (Can be made 1 hour in advance. Let stand at room temperature. Whisk before using.)
Whisk the remaining 1/3 cup cream and the mascarpone in another medium bowl until thick. Dice the cake into small cubes. Place about 1/2 cup of the cake cubes into each of 6 old-fashioned glasses or water goblets. Spoon 2 heaping tablespoons of zabaglione on top. Top with a dollop of the mascarpone mixture. Repeat the layers. (Wrap any leftover cake cubes in plastic wrap and freeze.) Top with white chocolate curls and serve.
Cook's Note: To make the chocolate curls, stand the chocolate bar in a large measuring cup. Microwave on 50-percent power in 10-second intervals just until the chocolate feels slightly warm. Using a vegetable peeler, scrape the edges of the chocolate bar to create curls.