Preheat the oven to 215°F. Line a baking tray with parchment paper. Fill a medium saucepan with about 2 inches of water and bring to a gentle simmer over medium heat.
Prepare the bowl of a standing mixer by thoroughly wiping the inside with a paper towel dampened with white vinegar. Add the egg whites, sugar, and cream of tartar. Place the bowl over the saucepan of gently simmering water, making sure the bottom of the bowl does not touch the water. Whisk constantly, until the sugar has completely dissolved, about 5-7 minutes. To check if it is ready, rub a bit of the mixture between two fingers; if you do not feel any grains of sugar, then the sugar has dissolved. Make sure to whisk constantly during this step because if left unattended, the bowl can become too hot and the eggs will start to cook.
Transfer the bowl to the standing mixer fitted with a whisk attachment. Whisk for about 10 minutes, starting on low speed and gradually increasing to high, until the whites increase in volume and form glossy, stiff peaks.
Drizzle the melted dark chocolate into your bowl and then turn out the entire mixture with a spatula onto your prepared baking tray. Spread the meringue into a rectangle. Try not to overwork or you will lose the lovely chocolate swirls in your meringue. Bake in the oven for approximately 1 hour 15 minutes. The longer you leave it in the oven, the drier your meringue becomes. I find after 1 hour and 15 minutes, the outside is crisp and the inside is soft.
Remove from the oven and allow your meringue to cool on the baking sheet placed on a wire cooling rack. While the meringue cools, whip the cream and sugar together in a large bowl until soft peaks form. Spread the whipped cream over the completely cooled meringue and top with the mixed berries.