Preheat oven to 180°C conventional oven (350° Fahrenheit/Gas 4). Grease a 24cm round cake pan by lightly buttering the base and sides with margarine or butter. Line the base with baking paper, then lightly butter over the baking paper. Add a heaped tablespoon plain flour (or cornflour), then turn and shake the tin to coat the butter in flour on the base and sides of the cake pan.
To make the Pan di Spagna (sponge cake), use an electric mixer or hand beater to beat the egg whites with a pinch of salt until stiff peaks form. With the mixer (or hand beater) running, add the egg yolks, one at a time. Add the sugar, one tablespoon at a time, beating well after each addition. Use a wooden spoon to stir in the cornflour, one tablespoon at a time. You want to see air bubbles rise to the surface of the cake mixture before adding additional tablespoons of cornflour. Stir the sifted baking powder into the cake mixture. Lastly add the lemon zest and stir until well combined.
Pour the cake mixture into the prepared cake pan. Bake for 30 mins or until a skewer inserted in centre comes out clean. Allow to cool in the cake pan for 10 minutes, then transfer to a wire rack to cool completely.
Meanwhile to make the Tiramisù cake filling, place the 3 egg yolks and 60g caster sugar in a large mixing bowl and beat on high speed until pale and thick. Add the mascarpone and beat on medium speed until combined, being careful not to over mix as you may split the mascarpone. Add the Marsala, then set aside.
To make the Italian meringue, pour the water into a clean, grease-free saucepan and slowly add the sugar, making sure all the grains get wet. Bring to the boil over medium heat and cook to soft-ball stage (when you can drop a small amount of syrup into cold water and it forms a ball that you can shape in your fingers, it is at 112°C on a sugar thermometer). Place the egg whites into a separate clean stainless-steel bowl. Beat the egg whites on high speed until they hold medium peaks. With the beaters still going, slowly and carefully add the hot syrup in a thin stream down the side of the bowl. Continue to beat until the meringue is completely cool. You know it is ready when the outside of the stainless-steel bowl no longer feels warm.
Gently fold the Italian meringue into the mascarpone mixture. Taste and add extra Marsala, if desired.
Transfer the Pan di Spagna (sponge cake) onto a serving plate. Slice the Pan di Spagna in half horizontally and spoon half of the cooled espresso coffee over the bottom cake half. Set aside the top half. Spoon the Tiramisù cake filling into a disposable piping bag fitted with a plain round nozzle. Pipe half of the filling on top of the coffee soaked Pan di Spagna. Use a small strainer to sprinkle unsweetened cocoa powder over the Tiramisù cake filling.
Note: The coffee shouldn’t soak all the way through – there should still be a little portion of cake left untouched.
Place the remaining cake on top, pressing down gently to secure it. Spoon the remaining espresso coffee on top of the cake. Pipe peaks of the Tiramisù cake filling on top of the coffee soaked Pan di Spagna, starting from the middle and working around in a circle until the top of the cake is covered completely. Top with a sprinkling of unsweetened cocoa powder – buon appetito.
Note: To save time, the Pan di Spagna (sponge cake) can be made the day before serving.