Pre heat your oven to 220ºc and evenly spread the caster sugar onto a lined baking tray. Cook for 5-6 minutes, until the edges of the sugar have started to caramelise.
In a clean bowl, whisk together the egg whites until stiff. On a medium spread, gradually whisk in the sugar a spoonful at a time and whisk for around 10 minutes until all of the sugar is combined and the meringue mixture is thick, glossy and holds a stiff peak.
Line a baking sheet, using a little of the meringue to hold down the greaseproof paper. Dollop generous spoonfuls of the meringue onto the baking tray, sprinkle with pistachios and bake for around 1 hour at 110ºc until the outside is crisp and the inside is still sightly gooey. Remove from the oven and cool.