REMOVE THE ZEST from the orange with a zester or grater and set it aside. Cut a flat bottom on the orange, then remove all the peel and white pith from the orange with a small, sharp knife. Cut the orange into 1 cm slices and refrigerate until serving time. Peel the mangoes and remove the pits. Puree the flesh with the orange zest in a food processor with the metal blade or in a blender until smooth. Strain through a fine mesh strainer into a 1.5-quart refrigerator container. Stir in the buttermilk, orange juice and honey to taste. The soup should be the consistency of whipping cream. If it is too thick, add more buttermilk. Chill thoroughly up to 2 days before serving. Just before serving, add lemon juice and more honey, if necessary. Pour into chilled dishes. Float an orange slice on top and garnish with a mint leaf. Makes 4 Cups