Preheat your oven to 180°C (160°C for fan ovens or gas mark 4). Line a 24 x 20cm brownie tin with baking parchment.

Melt the unsalted butter and dark chocolate over low heat, stirring until smooth. Stir in the orange zest and let cool slightly.

In a large bowl, whisk together the eggs and caster sugar until pale and fluffy, about 4-5 minutes.

Gently fold the chocolate mixture into the egg mixture until just incorporated.

Sift the plain flour and cocoa powder into the mixture and fold in gently until no lumps remain. Fold in the chopped dark chocolate orange.

Transfer the batter to the prepared tin, spreading it evenly. Bake for 35-40 minutes until the brownies rise and a skewer comes out with a few moist crumbs.

Let the brownies cool in the tin for 10 minutes before removing them to cool completely on a rack.