Step 1. Prepare a 20cm cake pan or loaf pan.
Step 2. Cream butter, orange rind and sugar until light and fluffy. Beat in egg yolks, one at a time.
Step 3. Add half of the sifted flour along with half of the yoghurt and half of the orange juice.
Step 4. Mix well, then repeat with the rest of the flour, yoghurt and juice. Beat the egg whites into soft peaks.
Step 5. Fold in a third of the egg whites into the cake batter to lighten the mixture and then fold in the remaining egg whites very lightly.
Step 6. Bake at 160C for approximately 1 hour.
Step 7. Combine the icing sugar, orange rind and butter and some of the juice. Add the remaining juice to give a good spreading consistency and apply the icing to a cold cake.