Preheat the oven to 170°c (150 fan/ Gas 3/ 325F) and line a 20 x 30cm baking tin with non-stick baking paper.
Place the butter and dark chocolate in a microwaveable bowl and melt in the microwave in 30 second bursts. Allow to cool.
Place the eggs in a large bowl and use an electric whisk to whisk for 2-3 minutes until light and fluffy.
Add the brown sugar to the eggs and whisk for another minute.
Stir the cooled melted chocolate mixture into the eggs mixture and stir to combine.
Gently mix in the flour and cocoa powder.
Fold in the chopped chocolate orange and orange zest.
Pour the mixture into the prepared tin and bake for 30-35 minutes until just set.
Allow to cool fully before cutting into bars.
Melt the dark chocolate for the topping and smear a little on the back of each chocolate orange minis segment before placing on top of each brownie bar.