Set aside one-fourth of the avocado. In a blender or food processor combine the remaining three-fourths of the avocado, the buttermilk, cream cheese, lime juice, garlic, the 1/8 teaspoon cayenne pepper and salt. Cover and blend or process until smooth. Coarsely chop the reserved avocado; stir into pureed mixture. Cover and chill in the refrigerator for 30 minutes to 24 hours.