Beat together the butter, sugar, treacle and orange zest. Mix in the egg.
Combine the flour, bicarb, salt, ginger, cinnamon and cloves. Divide the mixture in half. Add half of the dry mixture to the butter mixture and blend. Mix in the remaining half of dry mixture into the wet mixture until dough forms. Chill in the freezer for an hour or in the fridge for 2 hours.
Preheat the oven to 180 C / Gas 4. Grease a baking tray or line with parchment.
Roll out a portion of the dough to 5mm thick on a floured work surface. Keep remaining dough chilled whilst you work. Cut out shapes with a biscuit cutter. Place biscuits on prepared tray.
Bake for 8 to 10 minutes. Do NOT over-bake, or biscuits won't be soft. Cool on a wire rack.
For the icing: Blend all icing ingredients until combined. Use a piping bag to decorate gingerbread biscuits once they are completely cool.
If you don't have a piping bag for the icing, simply use a plastic freezer bag. Spoon icing into the bag, filling 1/3 full. Cut a small bit off one corner of the bag, and use to pipe onto the biscuits.