Mascarpone's silky thickness lends a custardlike texture without the trouble of cooking custard. Except for dissolving sugar in mango nectar, this is a no-cook recipe. Press the puree through a fine sieve for a smooth texture.
Combine nectar and sugar in a small saucepan. Cook over medium heat 5 minutes or until sugar dissolves, stirring constantly. Cool.
Place mango cubes in a blender, and process until smooth. Press pureed mango through a fine sieve over a bowl; discard solids. Combine mango puree, cheese, and sour cream in a blender; process until smooth. Pour into a bowl; stir in nectar mixture, juice, and salt. Cover; chill completely.
Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon ice cream into a chilled, freezer-safe container; cover and freeze 2 hours or until almost firm.