I just love a good zabaglione! This is another recipe, from my huge chocolate recipes cookbook, that I've made. It only uses a little chocolate, so it's best to use one with a minimum of 68% cocoa solids so that you get a good chocolate flavor. I use Ghiradelli and Valrhona.
In a large glass heatproof mixing bowl, whisk together the egg yolks and superfine sugar using an electric whisk, until you have a very pale mixture.
Fold the grated chocolate into the egg mixture. Gradually fold the Marsala into the chocolate mixture.
Set the mixing bowl over a pan of gently simmering water and set the electric whisk on the lowest speed or change to a hand-held balloon whisk. Cook gently, whisking constantly until the mixture thickens; take care not to overcook or the mixture will curdle.
Spoon the hot mixture into warm individual glass dishes or coffee cups and dust with unsweetened cocoa, if desired. Serve as soon as possible so that it is warm, light and fluffy.
Make the dessert just before serving because it will separate if you let it stand. If it starts to curdle, remove it from the heat immediately and place it in a bowl of cold water to stop the cooking. Whisk furiously until the mixture comes together.