Heat the oven to 110C/fan 90C/gas ¼. Start with the meringue. Whisk the egg whites to soft peaks with electric beaters, then start to slowly add the caster sugar 1 tbsp at a time, while whisking continuously on medium speed. Once all of the caster sugar is incorporated, add the icing sugar gradually, whisking until you have a stiff and glossy meringue. Line several baking sheets with baking paper then spread the meringue over the paper as thinly as possible and put in the oven to dry but not colour – this could take up to 5 hours, but check after 3 hours. It should feel dry and brittle. Cool and peel away the paper. Store in an airtight container.