In the bowl of a stand mixer or large mixing bowl, rub orange zest, ground cardamom and sugar together with the back of a spoon so the oils get into the sugar and you can smell the orange and cardamom.
Add the butter or margarine and mix until combined using a wooden ladle or mixer. Mix in egg yolk and vanilla essence.
Add the flour and salt then mix to form a smooth dough. Do not over-mix. Shape the dough into a disk and chill in the refrigerator for 30 minutes to an hour.
Preheat an oven to 180˚C/ 350˚F. Roll the dough to a thickness of 3/8-inch, between two sheets of parchment or baking paper.
To make the icing: combine icing sugar, orange juice and zest together. Whisk thoroughly till blended. Pour icing into a piping bag and seal the oven end. Thinly cut the closed end and squeeze over biscuits in a zip-zag fashion.
Biscuits not frosted can be stored in an airtight container at room temperature for seven days and unbaked cookie or biscuits dough can be kept in the freezer for three months.
Make your own caster or fine sugar by pulsing granulated sugar in a blender until the texture is fine.