Preheat oven to 120C. Whisk eggwhite and a pinch of salt in an electric mixer until firm peaks form (3-4 minutes). With motor running, gradually add caster sugar and whisk until thick and glossy (2-3 minutes). Sieve over icing sugar and cornflour, fold to combine, then spoon 8cm-diameter mounds onto oven trays lined with baking paper. Bake until meringues lift easily from trays and are crisp but not coloured (45-50 minutes), turn off oven and cool completely in oven. Meringues will keep stored in an airtight container for a week.