Preheat oven to 170°C. Line three oven trays with baking paper. Put eggwhites and sugar in a medium glass bowl over a saucepan of simmering water, making sure the bottom of the bowl doesn’t touch the water. Whisk until sugar dissolves, about 4 minutes.
Transfer mixture to the large bowl of an electric mixer and beat for 7-8 minutes or until mixture forms firm glossy peaks
Meanwhile, break half the chocolate into squares into a small heatproof bowl over a saucepan of simmering water, making sure the bottom of the bowl doesn’t touch the water, and stir until melted and smooth. Allow to cool slightly.
Finely chop remaining chocolate. Fold cocoa and chopped chocolate through meringue until combined. Add melted chocolate and fold through lightly, leaving a marbled effect.
Using a metal spoon, dollop mixture 2 inches apart on trays. Bake for no more than 15-17 minutes or until meringues are crisp on top with cracks, but have some give if pressed. (Note: overcooking for a few minutes will dry out centres.)
Allow to cool on trays for 10 minutes before carefully transferring to a wire rack to cool completely. Serve drizzled with extra melted chocolate if desired.