Preheat your oven to 170°C.

Butter a 20cm (8") springform pan and line the bottom and sides with parchment.

In a large heat proof bowl, combine the chopped chocolates and butter.

Combine the muscovado sugar and water in a small sauce pan and bring to a boil over medium heat, stirring frequently. Pour the boiling syrup over the chocolate and butter and stir until they have melted. Add the egg yolks, one at a time, to the chocolate mixture and then set the bowl aside until the mixture comes to room temperature.

In a large bowl, beat the egg whites and salt to a firm but not dry meringue.

Using a rubber spatula, fold a third of the meringue into the chocolate mixture. Once combined, add another third, fold and then fold in the remaining third until just combined.

Pour 2/3rds of the batter into your prepared pan (about 800g, reserving the remaining batter for later) and bake for 40 minutes, or until a skewer inserted comes out clean. Remove the cake from the oven and leave it on a wire rack to cool completely. Flatten the cake with an offset spatula. Don't worry about breaking the surface crust and pour the rest of the batter on top. Level the surface again. Return the cake to the oven and bake for a further 20-25 minutes. When tested with a skewer the cake should have a few moist crumbs clinging to it.

Allow to cool completely in the springform pan before attempting to unmold it. Dust with cocoa powder before serving.