Step 1 In a medium saucepan, heat the cream over medium heat just until it begins to bubble around the edges, about 5 minutes. Remove from heat. Whisk in the chocolate, and continue whisking until the chocolate is melted and incorporated with the cream to form a ganache (gently heat the cream a little more, if necessary, until the chocolate melts and mixes completely).
Step 2 Remove the ganache to a large bowl set over a bowl of ice and continue to whisk until the ganache chills and begins to thicken. Keep whisking until the ganache has a light,
Step 3whipped cream-like consistency. Remove the bowl from the ice bath and set aside.
Step 4In a large bowl, whisk together the egg yolks and sugar until combined and frothy. Whisk in the Port.
Step 5Set the bowl over a large pot of simmering water, making sure the bottom of the bowl does not touch the water. Continue to whisk until the mixture increases in volume and thickens to a thick, foamy cream, easily coating the back of a spoon. The consistency should be similar to that of a very thick hollandaise.
Step 6Remove from heat and gently fold the zabaglione into the whipped ganache. Place the mixture in a baking dish or bowl and cover the surface with a sheet of plastic wrap. Freeze until firm, about 1 1/2 to 2 hours. This makes a generous quart of frozen zabaglione.