Preheat the oven to 130ºC/250ºF/gas ½. Place the egg whites into a squeaky clean and dry bowl. Beat with an electric whisk on a medium speed until it forms stiff peaks. With your mixer still running, gradually add the caster sugar and salt, then whisk on a high speed for 7 to 8 minutes, or until white and glossy. To test whether it's done you can pinch some between your fingers – if it feels completely smooth it's ready; if it's slightly granular it needs a little more whisking. Line 2 large baking trays with greaseproof paper. Divide the mixture between each tray, then use the back of a spoon to shape and swirl it across the trays, leaving a 3cm gap around the edges. Bake in the oven for 1 hour 10 minutes, or until crisp on the outside and a little soft and sticky inside. Once cooked, leave the meringues to cool. Meanwhile, halve or quarter any large strawberries, then place the fruit in a bowl. Add 1 teaspoon of caster sugar, the vanilla bean paste and balsamic, stir well, then leave aside to macerate. Halve the vanilla pod lengthways, scrape out the seeds, then add to a large bowl with the cream and icing sugar. Whip until soft peaks form, then fold in the Greek yoghurt. Toast the nuts in a dry frying pan (if using) until golden, then lightly crush in a pestle and mortar. Gently melt the chocolate in a small pan over a low heat. Spoon half the vanilla cream over one of the cooled meringues and spread it out to the edges. Top with half the berries, half the melted chocolate and half the nuts. Layer over the second meringue and repeat with the remaining ingredients. Serve straightaway.