Cut avocados in half. Place a quarter of one avocado aside for garnish. Scoop out the rest of the avocado halves and place in a blender. Add lime juice, salt, Sugar and cumin and puree. Keep the blender going and add buttermilk slowly once the soup is pureed. Then add the cold water slowly. Chill and freeze for half an hour before serving. Cut the reserved quarter avocado in segments and decorate cool avocado soup.