Preheat the oven to 180C/160C fan/gas 4 and line a 24 x 20cm tin with baking paper.
Put the butter, dark chocolate and orange zest in a (non-stick) saucepan and gently melt over a low heat, stirring occasionally, until it's smooth – do not overheat. Then let it cool.
Whisk the eggs and sugar together with a electric whisk until the mixture is pale, has doubled in volume and leaves a little trail when the beaters are lifted. Stir it gently into the cooled chocolate mixture. Sift the flour and cocoa powder and stir in. Then add the orange chocolate.
Pour into the lined tin and bake for 35-40 mins. Cool in the tin, then cut into squares.