In a colander set over a large bowl, toss the shredded potatoes and onion and squeeze dry. Let the potatoes and onion drain for 2 to 3 minutes, then pour off the liquid in the bowl, leaving the starchy paste at the bottom. Add the potatoes and onions back into the bowl along with the eggs, scallions (if desired,) matzo meal, butter, salt, pepper, baking powder and mix well.

In a large skillet, heat a 1/8 inch layer of oil until shimmering. Spoon 1/4 mound of the batter onto the skillet about 2 inches apart and flatten slightly with a spatula. Fry the potato pancakes over moderately high heat, turning once until golden and crisp about 5-7 minutes. Transfer to paper towels to drain. Repeat with the remaining batter, adding more oil if needed. Serve with sour cream or creme fraiche, smoked salmon and a sprinkling of scallions or chives.