In a medium saucepan over medium heat, melt butter. Stir in curry powder and cook 1 minute. Stir in leeks, potato and apples and cook 5 minutes. Pour in broth and bring to a boil. Cover, reduce heat, and simmer 20 minutes. Puree in a blender, food processor, or an immersion blender. Season with salt and pepper and serve with a swirl of yogurt.