Place the potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 12-15 minutes or until tender. Drain and immediately place potatoes in ice water; drain and pat dry.
Peel two potatoes; finely dice and place in a small bowl. Cut the remaining potatoes in half. With a melon baller, scoop out pulp, leaving a 3/8-in. shell; set shells aside. Dice pulp and add to the bowl. Stir in the olives, parsley and onion. Combine the mayonnaise, mustard and pepper; gently stir into potato mixture.
Sprinkle potato shells with salt; stuff with potato salad. Sprinkle with paprika. Chill for at least 1 hour before serving. Garnish with parsley if desired.