A little preparation goes a long way with tiramisù. First,
prepare the soaking mixture for the savoiardi biscuits (you can
also use sponge cake if you like). It's important to use good,
strong coffee, preferably espresso; if it's weak, the flavour won't
carry through when you add the alcohol. I like to use a combination
of two types of Marsala: Boronia, a Marsala all'uovo, for its
sweet, creamy characteristics, and a drier style such as Pellegrino
Fine Marsala (you can use Boronia alone, but you may need to reduce
the sugar in the recipe). You can find both at good liquor shops. A
nip of the Pellegrino served on the side makes the perfect partner
to the dessert, too. And, although it's not traditional, I also add
a bit of rum and brandy for more of a boozy kick.
You want to make a fair amount of the soaking liquid because the sponge biscuits, true to their name, soak up a lot. Any leftover will keep in the fridge for a few weeks so you can whip up another batch of tiramisù down the track. Set aside the soaking mixture to cool while you get on with making the zabaglione, or sabayon.
Zabaglione is made by whisking egg yolks and sugar over gently simmering water with wine, usually sweet and in this case Marsala, added for flavouring. The mixture should be whisked constantly, but it doesn't need to be frantic - just keep it moving so that the mixture heats evenly. Run the whisk around the outer edges every now and then and scrape any splashes back down into the mixture so it doesn't burn.
The mixture will become thick and fluffy after a while, but you need to keep whisking until it's thick and stable, to a point where it tightens up and becomes more of a silky creamy consistency with a slight shine to it. The egg yolks have to reach 84C to absorb the alcohol, but it's hard to get a good reading on a thermometer because the mixture is so light and airy. Just dip your finger in and if it's too hot to hold it there, the mixture is ready; if not, keep whisking. The mixture should also hold a good trail when you drag the whisk through it, and look creamy; not foamy with small air bubbles. You're better off overcooking it than undercooking it. Let it cool before you fold in the cream.
When the zabaglione is cooled, add the mascarpone and cream mixture. I like to use Paesanella mascarpone, because it's locally made, and I mix it with cream to make the mixture a bit lighter. Break the mascarpone up a little first to loosen it so it combines more easily with the cream. The texture of mascarpone can vary - some are thicker and drier, while others are more wet. You want to whisk in the cream just until soft peaks form, but no further, so take care with a drier mascarpone because this can happen almost instantly.
Next, soak the savoiardi in the cooled liquid. Definitely cool the mixture first; the savoiardi soak up the mixture too quickly and fall apart if it's too hot. You could make your own sponge fingers, but I prefer using bought ones because they're more stable and soak up the liquid better. Dip the biscuits into the soaking mix for 10 seconds, then place them straight in the serving glasses or bowl to soften; they can be a little difficult to work with otherwise.
Adapt the layering to the size of the glasses you're using (small ones are perfect for guests who don't want to eat so much). The key is to have a good balance of soaked sponge and cream mixture, which is perhaps a matter of taste.
Finish the tiramisù with a dusting of cocoa and grated chocolate (use good dark chocolate of at least 53 per cent cocoa solids). Or, if you're feeling fancy, you can make chocolate curls.
Tiramisù is best made the day before so the flavours meld, but it also keeps well for a few days and the flavours become more melded as they sit and brew.