Set oven to 180C. Mix sugar, orange rind and egg until pale and creamy. Add oil, salt, yoghurt and orange juice, then mix well. Gently mix in flour and chocolate chips.
Spoon mixture into 12 muffin cups and bake for 15 minutes until springy on top.
Mix together icing sugar, melted butter, orange juice and rind and mix well to make a smooth icing.