Preheat the your oven to 180ºc/160ºc fan and line a 9″ square baking tray with parchment paper.
Melt together the butter, dark chocolate and chocolate orange in a heatproof bowl over a pan of boiling water, on the microwave for 1-2 minutes until melted.
Leave to the side to cool to room temperature
Whisk together the eggs and sugar for a few minutes until the colour has turned pale, is very mousse like, and is double the original volume
When whisked, pour the cooled chocolate mix over eggs and fold together carefully, making sure to not knock out the air
Once completely combined, add the cocoa powder and plain flour on top of the chocolate mix (sift if lumpy), add the orange zest, and then fold together again
Once combined well, fold through the chocolate orange chunks and pour into the prepared tin
Bake in the oven for 25-30+ minutes, until there is a small wobble in the middle of the tin and the edges of the brownie have set
Once baked, leave to cool in the tin completely. Set the brownies in the fridge for a couple of hours for best results.
Notes
These brownies last for 4-7 days, at room temperature, or in the fridge.