Whisk together the sour cream and chives in a small bowl. Set aside.
Preheat the oven to 200° F. Put a cooling rack on a baking sheet. Set aside.
Cut half of the potatoes into slices, place in a medium saucepan. Add ½ tablespoon salt, and cover with water. Bring to a boil over high heat, reduce the heat to low and simmer, uncovered, until the potatoes are fork-tender, about 10 minutes.
While the potatoes are cooking, grate the remaining potatoes using food processor or the large holes of a box grater. Transfer to wire strainer set over bowl. Toss with ¼ teaspoon of salt and squeeze by handfuls. Wrap potatoes in doubled paper towels and squeeze to remove as much liquid as possible. (See Tip 5)
Whisk together the flour, baking powder, 1 teaspoon salt, and ⅛ teaspoon pepper in a large bowl. Add the milk and egg, and whisk until smooth.
Drain the boiled potatoes and return them to the pan. Add the half-and-half and 3 tablespoons of the butter and mash until the potatoes are smooth. Give the grated potatoes one final squeeze and then stir them into the mashed potatoes. Stir the potato mixture into the flour-milk mixture.
Heat a 12-inch nonstick skillet over medium heat. Add one tablespoon of canola oil and one tablespoon of butter; heat until it begins to shimmer. Working in batches, drop scant ¼ cups of the batter into the skillet and spread the batter into ¼-inch-thick rounds. Cook until the Boxty bottoms are golden brown, 4 to 5 minutes. Flip and cook the other side until golden brown, about 3 minutes longer. Transfer to paper towel lined plate to drain, and then to the prepared baking sheet. Keep warm in the oven while you repeat with more oil, butter and batter. (See Tip 6)
Serve the Boxty warm, topped with a dollop of the sour cream and chive sauce and slices of smoked salmon. Garish with chopped chives.
Yield 4 to 6 servings.