Preheat oven to 325°. Peel potatoes. On the grater disk on a food processor, grate potatoes and onions. Transfer to a large kitchen towel.
Gather ends of towel; twist over sink and squeeze firmly to wring out as much liquid as possible. Open towel, toss mixture to loosen. Gather towel and wring out once more. This is the toughest but most important step!
Whisk eggs, breadcrumbs, salt, baking powder, and pepper in a medium bowl. Add potato mixture. Mix until well coated. It should be wet and thick but not soupy.
Set a wire rack inside another large rimmed baking sheet; set aside.
Heat about 2 tablespoons oil (I threw some butter in there too, why not) in a 12 inches nonstick skillet over medium-high heat. Drop a small amount of latke mixture into pan. Smoosh the center down a little so they fry evenly. You don't want them too be too chubby.
Working in batches and adding more butter and oil to skillet as needed, drop spoonfuls of mixture into pan, pressing gently with the back of a spoon or spatula to flatten slightly. (If mixture becomes watery between batches, mix to incorporate)
Cook latkes, occasionally rotating pan for even browning, until golden brown and cooked through, 2-3 minutes per side. Transfer to paper wire rack to drain then place sheet with latkes in oven to keep warm and crisp while cooking remaining latkes.
Serve latkes while warm with applesauce and/or sour cream.