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Heat oven to 180C/160C fan/gas 4 and line a 24 x 20cm brownie tin with baking parchment. Put the butter, dark chocolate and orange zest in a non-stick saucepan and very gently melt over a low heat, stirring every now and then, until smooth – take care not to overheat it. Cool.
Whisk the eggs and sugar together with an electric whisk until the mixture is pale, has doubled in volume and leaves a trail when the beaters are lifted. Gently stir into the cooled chocolate mixture. Sift over the flour and cocoa, 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.