Requires: 23cm square cake pan (alternatively use 24cm round baking pan), electric mixer or hand beater
Preheat oven to 180°C conventional oven (350° Fahrenheit/Gas 4). Grease the 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 of plain (all-purpose) flour, then turn and shake the tin to coat the butter in flour on the base and sides of the cake pan.
To make the 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 in the sifted baking powder and lemon zest until well combined.
Pour the cake mixture into the prepared cake pan. Bake for 25 mins or until a skewer inserted in centre comes out clean. Keep the oven on to cook the puff pastry.
Meanwhile to make the Crema Diplomatica, heat the milk in a medium-sized saucepan over low to medium heat until just before boing point, then remove from the heat. Place the egg yolks and caster sugar in a large stainless-steel bowl. Whisk until foamy and creamy then slowly whisk in the flour, one tablespoon at a time, until well combined. Continue to whisk the egg mixture and slowly add the warmed milk until just combined. Transfer to the saucepan over low heat. Add the lemon zest and continue to whisk until thick (it should be thick enough to coat the back of a spoon). Remove from the heat and place into an ice bath to quickly cool – refer to Note in this step for instructions.
Note: To make an Ice Bath, place enough cold water into the sink to halfway cover the sides of the saucepan. Add ice cubes.
Meanwhile, place the thickened cream into a bowl. Add the sifted icing sugar and vanilla bean seeds (or vanilla bean paste). Use an electric mixer or hand beater to beat until stiff peaks form. Use a spatula to fold the whipped cream mixture into the vanilla custard, a little at a time, until well combined and smooth. Refrigerate until ready to use.
To make the Strega syrup, combine the Strega liqueur, water and sugar in a small saucepan over low-medium heat. Bring to the boil, then remove from the heat. Set aside to cool.
Note: Strega is one of our favourite Italian herbal liqueurs. It contains approximately 70 herbal ingredients including mint and fennel. The presence of saffron is what gives Strega its yellow color. While we have chosen Strega for this recipe for the flavour, you could use another liqueur of your choice e.g. rum.
To make the two puff pastry layers, line a large baking tray with baking paper. Place the two puff pastry sheets on the prepared baking tray. If the layers of puff pastry are too thick, sift icing sugar (confectioners’ sugar) over the pastry and use a rolling pin to roll thiner. Use a fork to prick the pastry all over. Sift icing sugar (confectioners’ sugar) to cover the puff pastry. Place a sheet of baking paper on top of the pastry and place an empty baking tray on top as a weight. Bake for 25 minutes, then remove baking tray weight and baking paper. Bake for a further 5 minutes uncovered or until the puff pastry is golden brown. Remove from the oven and allow to cool completely.
Using a serrated knife, cut the sponge cake to a square shape, 22 x 22cm in size. Use the same knife to trim the two puff pastry sheets to the same size as the cake.
Line a 22cm square pan or dish with aluminium foil, so that the foil comes up over the sides (this allows you to lift out the Italian vanilla slice). Place one sheet of the puff pastry sheet on the bottom of the aluminium lined pan/dish.
Note: If you don’t have a 22cm pan, you can still create the layers on a serving plate. You could even pipe the Crema Diplomatica for a professional finish.
Pour half of the Crema Diplomatica over the puff pastry layer. Place the sponge cake on top of the Crema Chantilly layer. Using a pastry brush, generously soak the sponge using all of the Strega syrup. Top the sponge with the remaining Crema Diplomatica. Top with the remaining puff pastry sheet. Refrigerate for minimum of 2 hours before serving.
Carefully lift the Italian vanilla slice from the pan. Generously top with sifted icing sugar. Use a serrated knife to cut into four squares to serve – buon appetito!