Preheat oven to 110 C (fan-forced). Line two baking trays with baking paper. Sift the cocoa powder into a small bowl. Set aside.
Using clean, dry electric beaters whisk the eggwhites and cornflour in a medium bowl until firm peaks form. Add the sugar, 1 tablespoon at a time, whisking well between each addition. Once all the sugar has been added, whisk for 2 mins.
Add the vanilla essence and whisk until combined. Add the cocoa and fold in until just combined. Spoon tablespoonfuls (or whatever size you would like to make them, or you can put the mixture into a piping bag and use a plain or star shaped tube to pipe), onto the lined trays about 3cm apart.
Bake meringues for 1 ¼ - 1 ½ hours or until the meringues sound hollow when tapped on the bottom. Turn the oven off and leave the meringues in the oven to cool. Once cool, transfer to an airtight container. Dust with cocoa powder, to serve, if you like.
Keep the meringues in an airtight container for up to 1 week.