Grate the potatoes and onions on the large holes of a box grater or on the grating disk of a food processor. Transfer the mixture to a cheesecloth or a clean, lint-free dishtowel and squeeze out as much of the liquid as possible. You’ll be surprised how much liquid comes out.
In a large mixing bowl, whisk together flour, baking powder, salt, and pepper then whisk in the egg. Add in the squeezed dry potatoes and stir until the potatoes are evenly coated in the batter.
Heat a large, heavy-bottomed skillet over medium-high heat and add about 1/4” of oil (enough to generously coat the bottom of the pan). Once the oil shimmers, add the batter to the pan 1 heaping tablespoon at a time (or use a trigger-release ice cream scoop) and immediately press with a spatula to form disks. Fry until the edges are browned, 4-5 minutes. Flip and cook until the second side is golden brown, about 3 to 4 minutes more.

Transfer latkes to a paper-towel-lined plate and immediately sprinkle lightly with kosher salt.
Repeat with the remaining potato mixture adding more oil to the skillet as needed. Serve warm latke with sour cream garnished with chives.