1.Heat oil in a large saucepan over moderate heat. Add onion; cook, stirring, 5 minutes or until softened. Add garlic, zest and cumin; cook, stirring 1 minute or until fragrant. Add zucchini; cook, stirring, for 1 minute, then add chickpeas, crumbled stock cubes and water. Bring to the boil. Reduce heat; simmer 3 minutes or until zucchini is just tender. Remove from heat. Cool slightly.
2.Add chives and parsley to soup. Blend or process in a blender, in batches, until smooth. Stir in half the yoghurt. Transfer soup to a bowl. Cover, then refrigerate until well chilled.
3.To serve, stir soup until well combined, then divide among serving bowls. Serve topped with remaining yogurt and extra chives.