Peel potatoes and coarsely shred using the coarsest side of the grater (you should have about 2 cups). Rinse shredded potatoes in a colander; drain well and pat dry. In a medium bowl combine shredded potatoes, onion, jalapeno pepper (if using), salt, and black pepper.
In a large nonstick skillet heat butter over medium heat for 1 to 2 minutes or until hot. Carefully add potatoes, pressing into an even pancake-like round (about 7 to 8 inches in diameter). Using a spatula, press mixture firmly. Cover and cook over medium heat for 8 minutes or until golden brown. Check occasionally and reduce heat, if necessary, to prevent overbrowning. Turn using two spatulas or a spatula and fork. (If you're not sure you can turn in a single flip, cut into quarters and turn by sections.) Cook, uncovered, 5 to 7 minutes more or until golden brown and crisp. Remove from skillet; cut into wedges. Makes 2 or 3 servings.