Instructions: In a food processor fitted with coarse shredding disk shred boiling potato and transfer to a bowl of cold water, swishing it around to remove excess starch. Drain potato in a colander and pat dry with paper towels. Peel sweet potato and shred in food processor.

In a bowl toss together the shredded potatoes. In a 10" nonstick skillet heat 1/2 tablespoon oil and 1/2 tablespoon butter over moderate heat until foam subsides. Spread potatoes evenly in a skillet and season with salt and pepper. Cook potato cake, tamping down firmly with a spatula, until underside is golden, about 5 minutes. Reduce heat to moderately low and cook 5 minutes more.

Slide cake onto a large plate. Invert another large plate over cake and invert cake onto it. Heat remaining oil and butter over moderate heat until foam subsides and slide cake back into skillet. Season cake with salt and pepper and cook until underside is golden and cake is cooked through, about 5 minutes. Cut cake into 4 wedges.

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