For the mango sauce, combine mango, pineapple juice, lime juice, and granulated sugar in a blender or food processor. Cover and blend or process until smooth. Spoon into a serving bowl, cover, and chill until serving time or up to 24 hours.
For each wonton, lay a wonton wrapper on a work surface with a point toward you. Lay a banana slice and a chocolate piece on the lower half of the wonton wrapper. Brush edges of wonton wrapper with water. Fold the top point down to meet the bottom, forming a triangle. Gently press the edges together to seal. Set aside; cover with plastic wrap while filling remaining wonton wrappers. Repeat with remaining wonton wrappers, banana slices, and chocolate pieces. (At this point, filled wontons can be covered with plastic wrap and chilled up to 24 hours before frying.)
Heat 3 inches of cooking oil in a deep-fryer or large saucepan to 365 degrees F. Fry wontons, 4 to 6 at a time, in hot oil for 2 to 3 minutes or until golden brown, turning occasionally to brown evenly. Use a slotted spoon to remove wontons from oil to paper towels to drain. Cool about 5 minutes. Sprinkle with powdered sugar and serve within 1 hour with mango sauce for dipping. Makes 24 wontons.