Preheat the oven to 170°C.
Grease a 20 cm springform cake tin and line the base ans sides with baking parchment
Place the butter and the chocolates in avery large heatproof bowl .Put the brown sugar and water in a small saucepan, stir to mix, then bring to the boil over a medium heat.
Pour the boiling syrup over the butter and chocolate and stir well until they have melted, add the grated zest orange and you are left with a runny chocolate sauce. Stir in the egg yolks, one at time. Set aside until the mixture comes to room temperature.
Putthe egg whites and salt in alarge bowl and whisk to a firm, but not too dry meringue. Using a rubber psatula or a large metal spoon, gently fold the meringue into the cooled chocolate mixture a third at a time. The whites should be fully incorporated .
Pour about two-thirds of the mixture into the prepared cake tin and level gently with a palette knife. Leave the rest of the batter for later. Placethe cake in the oven and bake for about 40 minutes, until a skewer inserted in the centre comes out almost clean. Remove from the oven and leave to cool completely.
Flatten the top of the cake with a palette knife. Don't worry about breaking the crust. Pour the rest of the batter on top and level the surface again. Return to the oven and bake for 20 /25 minutes. The cake should still have moist crumbs when check with the skewer. Leave to cool completely before removing from the tin . Dust cocoa powder and serve.
The cake will keep, covered, at room temperature for 4 days.