In a large saucepan, melt the butter over medium heat. Add the scallion whites, bell pepper and curry powder and cook, stirring, until tender, about 3 minutes. Add the chicken broth, cauliflower and potato and bring to a boil. Lower the heat, partially cover and simmer until the vegetables are tender, 10 to 12 minutes.
Stir in the chicken and chickpeas and cook until heated through, about 2 minutes. Stir in the scallion greens and season with salt and pepper. Stir in the yogurt just before serving.