Crush the cardamom pods using a pestle and mortar or a small bowl and the end of a rolling pin. Discard the shells and crush the seeds as finely as possible.
Put the flour and crushed seeds in a bowl or food processor. Add the butter and rub in with the fingertips or process until the mixture resembles coarse breadcrumbs. Add the icing sugar and stir in or blend briefly. Add the egg yolk and orange rind and mix or blend to a dough. Wrap in clingfilm and chill for 1 hour.
Roll out the dough on a lightly floured surface to 5 mm (¼ inch) thick. Cut out about 12 heart shapes using a 5–6 cm (2–2½ inch) heart biscuit cutter, re-rolling the trimmings to make more. Place slightly apart on a large greased baking sheet and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for about 15 minutes until pale golden. Transfer to a wire rack to cool.
Make the icing. Beat together the icing sugar and orange juice in a bowl to make a smooth, thin icing. Drizzle lines of icing over the biscuits to decorate.