Cut the butter into small cubes and tip into a bowl. Break dark chocolate into small pieces and drop into the bowl. Fill a saucepan about a quarter full with boiling water then sit the bowl on top so it rests on the rim of the pan, not touching the water. Put over a low heat until the butter and chocolate have melted, stirring occasionally to mix them.
Remove the bowl from the pan. Leave the melted mixture to cool to room temperature.
While you wait for the chocolate to cool, position a shelf in the middle of your oven and turn the oven on to 180 C / 160 C fan / Gas 4.
Chop the white chocolate into large chunks and zest the oranges.
Break eggs into a large bowl and tip in the golden caster sugar. With an electric mixer on maximum speed, whisk the eggs and sugar. They will look thick and creamy, like a milkshake. This can take 3 to 8 minutes. If the mixture that runs off the beaters leaves a trail on the surface of the mixture in the bowl for a second or two, you’re there.
Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a wooden spoon.The idea is to combine them without knocking out the air, so be as gentle and slow as you like.
Hold the sieve over the bowl of eggy chocolate mixture and resift the cocoa and flour mixture, shaking the sieve from side to side, to cover the top evenly. Gently fold it in, using the same figure of eight action as before. Add the white chocolate chunks and orange zest and fold in as well.
Pour mixture into 40x25cm baking tin and bake in the preheated oven until the brownie starting to come away from the edges of the tray and it is not wobbling in the centre, about 25 minutes.
Remove from oven and place on a cooling tray. Leave the whole thing in the tin until completely cold. Remove and cut into portions.
To decorate, drizzle with melted white chocolate if desired.