Preheat the oven to 100 C / Gas 1/4. Line 2 baking trays with baking parchment.
Gently tip the egg whites into a large mixing bowl and use an electric hand whisk to beat the egg whites until they are fluffy and stiff peaks form when the whisk is lifted.
Continue beating with the electric whisk and add the caster sugar a tablespoonful at a time. Beat for about 5 seconds between each addition. It is vital that the sugar is added slowly to ensure that the meringue does not weep whilst in the oven. However, try not to over-beat. The meringue mixture should be thick and glossy. Remove the whisk.
Sift 1/3 of the icing sugar over the mixture and gently fold it into the mixture using a rubber spatula or a large spoon. Continue to sift and fold in the icing sugar 1/3 at a time. Again, try not to over-mix. The meringue mixture should be smooth and light. If you're brave enough, tip the bowl upside down and the mixture should (hopefully) stick to the bowl and not drip out!
Scoop up a heaped tablespoonful of the mixture and drop it onto the pre-lined baking tray. Repeat this, placing the meringue mixture roughly 2cm apart on the trays. If you like a peaked tip, gently lift the centre of the meringue with the end of a spoon.
Bake in the preheated oven until it is a light coffee colour and it sounds 'crisp' when tapped underneath, about 1 3/4 hours. When you are sure the meringue is cooked, remove from the oven and allow to cool on the trays. Serve to your liking!