Fill a medium-sized pot with water and bring to a boil over high heat. Add mini-red potatoes and boil until fork tender, about 20 minutes. Drain and let cool.

Meanwhile, fill another medium-sized pot with water and bring to a boil over high heat. Add sweet potatoes and white potato and boil until tender, about 20 minutes.

In a medium bowl, combine sweet potatoes, white potato, margarine, salt, and pepper. Using a potato masher, mash until smooth. Insert mixture into a pastry bag fitted with a star or flower tip.

Slice off the top of each red potato (see photo). Using a melon baller, scoop out the center of each potato. Pipe some of the sweet potato mixture into the centers. Sprinkle with fresh parsley and serve.