Right before serving in a food processor puree the broth, garlic, lime juice and 1 of the avocados; season well to taste with salt and cayenne pepper.
Add ice water to thin the soup to the texture you like. Chill the soup while you peel and cut remaining avocado into small dice. Fold the diced avocado into the base and adjust the seasoning.
To serve, ladle soup into chilled bowls and garnish with cilantro, chives and a dollop of yogurt in the center or each portion.