Set aside some of the mango so you have about 1/2 cup diced and reserved for garnish.
Roughly chop the remaining mango and puree it in a food processor with the buttermilk, orange rind, and juice. Season to taste with salt and pepper.
Cover and chill overnight. Adjust the seasoning before serving. Garnish with reserved diced mango, scallion and a dollop of sour cream centered in the middle.