Working in batches, blend avocados, chicken broth, 2/3 cup milk, chopped shallots, 2 tablespoons Sherry and 1/2 teaspoon hot pepper sauce in blender until mixture is smooth. Season soup to taste with salt and pepper. Cover and refrigerate until cold. (Can be made 6 hours ahead. Keep refrigerated.)Ladle soup into bowls. Spoon dollop of sour cream atop soup.
