Preheat the oven to 170 degrees celsius (340 Fahrenheit). Place a heatproof bowl over a saucepan of simmering water and melt the chocolate and coconut oil. Stir to combine and leave to cool for 5 minutes before moving onto the next step.
Add the eggs and brown and white sugars to the bowl of an electric stand mixerfitted with a whisk attachment. Whisk until thick, light and creamy, around 5 to 7 minutes. The mixture should form thick creamy ribbons when you lift the whisk, that sit on top of the mixture for a few seconds before settling into the rest of the mixture. If it doesn't do this keep whisking.
Add the melted chocolate mixture and sift in the cocoa and baking soda. Fold through the eggs before adding the extra chopped chocolate and orange zest.
Drop spoonfuls of the batter onto 2 baking sheets lined with baking paper and bake for 8 minutes. Leave on the tray to a 5 minutes to firm up before transferring to a wire rack to cool completely.