I couldn't get the cream chocolate mixture to whip to the "fluffy" stage. I thought it was cold enough after an hour in the fridge but maybe not.

Semifreddo is one of my favorite desserts to make - thinking of doing this one next weekend for a dinner party. I notice that the recipe lists cream, but not specifically heavy cream - maybe SK Mazza used light cream (aka single cream)? That would explain it not whipping properly. Semifreddo really is essentially a frozen mousse.

Hi, it should be cold enough. And it shouldn't take too long to whip the mixture until fluffy (think whipped cream but thicker, like a mousse). This stage, along with the meringue, is important because adding the air to the mixture via whipping is partly what makes the semifreddo so light (and not a frozen solid block like an ice cube). Depending on the type of cream used it may not whip up quickly but the texture you are looking for is that of a mousse. Hope it turns out well in the end for you!