1. Preheat the oven to 200°C/180°C fan/Gas 6. To make the genoise, put the eggs, sugar, zest and vanilla into a heatproof mixing bowl and set over a pan of barely simmering water. Whisk together with a handheld electric mixer until the mixture feels slightly warm, then remove from the heat and continue whisking until the eggs have tripled in volume and you have achieved the ribbon stage – when you lift the whisks out of the mixture, the batter drips down and sits proud on the surface for at least 4 seconds. If you have a freestanding electric mixer, simply do it in that on full speed – much quicker, and no need for the warm water.
2. Sift the flour over the surface and very gently fold in, ensuring no clumps of flour lurk anywhere. Carefully pour the batter into the prepared cake tin, and bake for 20–25 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in the tin until completely cold.
3. Make the soaking syrup by mixing together the water and sugar. Bring to the boil in a small saucepan and allow to bubble for 2 minutes. Remove from the heat and allow to cool before stirring in the orange juice.
4. For the Italian meringue topping, put the egg whites in a heatproof mixing bowl – preferably metal – and whisk using a handheld electric mixer. You could also do this in a freestanding electric mixer with whisk attachment. Beat the egg whites until they are at soft peaks: when you lift the whisk out of them they just hold a peak, albeit a floppy one. When you reach this stage, stop mixing.
5. Place the sugar and water in a medium saucepan over a medium-high heat, and clip the sugar thermometer in place. If your thermometer doesn’t have a clip, simply hold it. Do not stir the mixture at any point, just leave it to boil and bubble away. When the thermometer reads 118°C or ‘softball stage’ – after about 5 minutes – remove the pan from the heat immediately, being careful not to touch the sugar mixture because it’s extremely hot. Turn the mixer for the egg whites back on to medium speed and continue mixing as you gently pour the sugar syrup down the side of the bowl. Continue whisking on medium until the meringue has cooled to room temperature. This will take about 6 or 7 minutes.
6. To finish, take a sharp knife and run it around the edge of the cake tin to release the cake. Slice the cake in half horizontally as evenly as possible. Place what was the top half of cake cut-side-up on a cake stand and soak it well with some of the syrup. Scoop the meringue into the piping bag and pipe a generous swirl on top. Soak the cut side of the other cake half, and place it, cut-side-down, on top of the meringue. Spoon pillowy blobs of the meringue on top of the cake, then arrange the fruit as desired. Finish with a light dusting of icing sugar.