How to Make It
In a small skillet, toast the rice over moderate heat, shaking the pan frequently, until lightly browned and fragrant, about 10 minutes. Let cool slightly, then transfer to a spice grinder and coarsely grind. Sift to remove any big pieces; you should have 1 tablespoon.
Wipe out the skillet. Add the dried chiles and toast over moderate heat until browned in spots, about 3 minutes. Transfer to the spice grinder and grind to a coarse powder; some seeds will remain.
In a medium bowl, stir the fish sauce with the sugar until the sugar dissolves. Stir in the shallot, fresh chile and toasted rice powder. Add the dried chile powder, 1 teaspoon at a time, checking the heat as you go. Let the dipping sauce stand for 20 minutes. Transfer to a small bowl and serve with the mangoes and lime wedges.