1. Separate the egg whites from the yolks, saving the yolks for another purpose.
2. Place the egg whites in a very large, clean mixing bowl.
3. Weigh the caster sugar.
4. Line a flat baking tray with a non-stick mat or non-stick baking paper.
5. Pre-heat the oven to 100°C (Gas Mark ¼).
6. Using a handheld electric whisk, whisk the egg whites until they begin to form soft peaks.
7. Turn the whisk up to maximum speed and slowly begin to add the sugar a little at a time until all the sugar has been incorporated such that the meringue mix is nice, stiff and quite shiny.
8. Fill a piping bag fitted with a large star tube with the mixture and neatly pipe out into about 8cm lengths onto the baking tray.
9. Wiggle the piping bag slightly as you go, which will give you a wider meringue and a pleasant shape such as circles.
10. Put the meringues in the pre-heated oven and allow to bake very slowly for about 2 hours, until crispy and, with a bit of luck, a slight coffee color.
11. Turn the oven off completely and allow the meringues to cool while still in the oven.
12. When required, whip the cream until it is thick enough to pipe.
13. If you are using them, chop the pistachio nuts and melt the chocolate in a bain marie.
14. Fill a piping bag fitted with a star tube with the whipped cream and carefully pipe some cream all over the flat-bottom side of one of the meringue shells.
15. Gently sandwich together with another meringue shell and sprinkle over some chopped pistachios.
16. Spoon a little melted chocolate into a small paper piping bag, snip off the tip and neatly pipe the chocolate across the meringue.
17. Place in the fridge for a few moments to allow the chocolate to set.
18. To finish, place the meringues in a nice paper case (muffin cases are ideal) and serve.