For the sponge filling: put the ladyfingers into a large dish or baking tray, in a single layer. Mix together the water and limoncello, then pour it over the ladyfingers. Put the dish into the fridge while you prepare the other ingredients.
For the cream filling: whisk the egg whites until they form firm peaks. Put the egg yolks and sugar into a separate bowl and whisk them until light and fluffy. Put the mascarpone, double cream and lemon zest into a third bowl, and whip together until thick.
Pour the egg yolk mixture into the cheese and cream bowl, and mix it all together with a spoon or spatula. Add the egg whites and gently fold them through, taking care not to knock much air out.
To assemble the tiramisus: use a cookie cutter to cut a circle of ladyfingers, the same size as your sundae glasses. Put a layer of ladyfingers in the bottom of each glass, then top with a few generous spoonfuls of cream cheese filling. Add another layer of sponge, then another layer of cream cheese filling. Continue until the serving glasses are full to the top - aiming for at least four layers.
Put the serving glasses into the fridge to chill for one hour.
To make the meringue: put the caster sugar and water into a small saucepan and stir until the sugar has melted, then bring to the boil. Leave the saucepan without stirring, until it reaches a fast boil. Use a sugar or digital thermometer to check the temperature of the sugar syrup - it needs to reach 120C.
Meanwhile, put the egg whites and lemon juice into the bowl of a stand mixer and whisk until it reaches soft peaks.
Once the sugar has reached 120C, with the stand mixer still running, slowly pour the sugar syrup into the whipped egg whites. Keep whisking until the meringue has formed soft peaks and is cool, then switch off the mixer.
Spoon a generous pile of meringue on top of each sundae glass, and use the back of a spoon to shape it into a meringue cloud shape.
Use a blow torch to brown the meringue. Start slowly, taking care not to burn it, until you get the hang of it.
Serving the tiramisus: serve the tiramisus not long after you’ve browned the meringue. If you want to make them in advance, you can keep the tiramisus in the fridge and keep the meringue in the bowl of the stand mixer. Re-whip the meringue for five minutes before using it to top the desserts, just before serving them.