Preheat the oven to fan 160C/conventional 180C/gas 4. Sift an even layer of icing sugar over a plate and lightly whisk the egg white in a saucer. Pick off small coriander sprigs and brush each side lightly with egg white using a small paint brush. Gently drop the sprigs flat on to the icing sugar, then sprinkle lightly with white caster sugar. Lay them out flat on a baking sheet lined with non-stick silicone (we like Bake-O-Glide) or plastic sheeting. Bake for just a few minutes until the sugar melts and the sprigs begin to puff. Immediately remove and leave to cool until crisp. They will keep in an airtight tin for up 3 days.