Preheat the oven to 180℃/355℉. Line a baking tray with baking paper.
In a large bowl whisk the egg whites until soft peaks form. Then gradually add spoonful's of castor sugar, whisking until thick and glossy with stiff peaks.
Add cocoa powder, corn starch, vanilla essence and vinegar. Whisk for 10 seconds or until combined. Note: make sure to scrape the sides to ensure the ingredients are well combined.
Fold in the chopped chocolate. Dollop the chocolate meringue on the lined baking tray. It should be quit stacked about 25cm radius x 10cm high. Smooth the edges and surface (as shown in the video).
Place in the oven and immediately lower the oven temperature to 150℃/300℉. Bake for 1hr 15 minutes. Do not open the oven door during this time. The meringue will expand in the oven and crack, this is normal. Once it's done baking, turn the oven off but do not open the door. Allow the meringue to cool in the oven for 3 hours or overnight.