Special equipment: ice cube trays
Put the sugar, lemon juice and mangos in a blender and blend until liquified and smooth. The mixture should be pourable; if it is too thick, blend in water 1 tablespoon at a time. Pour the mango juice into ice cube trays and freeze until solid, about 2 hours.
To serve, place 2 to 3 ice cubes in the bottom of a champagne flute and fill with prosecco.