Preparation Scrape avocados from skin into processor. Add buttermilk and yogurt; purée until smooth. Mix in lime juice, jalepeño and chili powder. With machine running, blend in 1/2 cup broth. Season with salt and pepper. Chill soup until cold. (Can be made 1 day ahead. Cover and keep chilled.) Thin soup with more broth, if desired. Spoon into bowls. Sprinkle with onion.
Preparation Scrape avocados from skin into processor. Add buttermilk and yogurt; purée until smooth. Mix in lime juice, jalepeño and chili powder. With machine running, blend in 1/2 cup broth. Season with salt and pepper. Chill soup until cold. (Can be made 1 day ahead. Cover and keep chilled.) Thin soup with more broth, if desired. Spoon into bowls. Sprinkle with onion.