For the biscuits, beat the butter and sugar and orange zest together in a bowl until light and fluffy.
Lay out a sheet of cling film on a clean work surface.
Add the egg to the mixture and beat together. Add the flour to bring together as a dough.
Wrap the dough in cling film and refrigerate it for at least 15 minutes.
Preheat the oven to 180C/350F/Gas 4.
Flour the work surface and then roll the dough gently, using a floured rolling pin, until about 0.5cm/¼in thick.
Cut out the biscuits using biscuits cutters of your choice.
Transfer to a baking tray using a palette knife, ensuring the biscuits are well spaced apart. Bake for 10-15 minutes, or until lightly golden-brown.
Remove to a wire rack to cool.
For the icing, sift the icing sugar and divide it between two bowls. Sift the cocoa powder into one of the bowls.
Gradually add the liquid to each bowl, a teaspoon at a time, until the icing is thick but runny. If it is too runny, add more sifted icing sugar.
Add a drop of orange food colouring to the white icing.
Put the icing into two separate piping bags. Pipe orange and brown patterns onto the biscuits. Leave to harden to serve.