Barley and yogurt soup (tanabour)

Drain barley and place in a large saucepan with 2 litres water and 1 tsp salt. Bring to the boil over medium heat and cook, skimming any impurities that rise to the surface, for 35 minutes or until barley is tender. Remove from heat.
Whisk yoghurt and eggs in a bowl until combined. Gradually whisk in 125 ml hot soup, then whisk yoghurt mixture back into saucepan of remaining soup.
Return soup over medium heat and cook for 5 minutes or until heated through; do not allow soup to boil. Melt butter in a small saucepan, then stir in mint and pour over soup to serve.