Pre heat your oven to 100℃ (212℉) -use convention setting, not fan-forced (see notes)
Line a baking sheet with silicone baking mats or baking paper (I use 2 large baking sheets)
In the mixer bowl of a stand mixer, place the egg white and cream of tartar
Using a hand mixer on low to medium speed, whisk the egg whites until foamy and beginning to form soft peaks (I use the hand mixer first, as it's a small amount of egg white and it's harder to whisk with the stand mixer at the start)
Place your mixing bowl onto your stand mixer and using a whisk attachment, whisk on low to medium, while slowly adding the caster sugar to the whipped whites, 1 tablespoon at a time (leave 30 seconds between each spoonful)
Once all of the sugar has been added, stop the mixer and scrape the side of the bowl with a silicone spatula
Now mix the meringue on medium speed for about 3 minutes, until it forms stiff peaks and is nice and glossy. Check that there are no large granules left, by rubbing a little of the meringue between your thumb and finger. You want a glossy, marshmallowy meringue here
Transfer mixture to a piping bag fitted with a star nozzle, then pipe out your meringue cookies. Because you're going to crush these, they don't need to be fancy. Pipe about 8 meringue cookies
Bake in the oven for 2.5 hours (for crispy, hard meringues) or 1.5 hours for a softer centre, then turn off the oven and leave the meringues to cool, with the oven door slightly ajar