Position a rack in the middle of the oven and preheat to 425 degrees. Place a towel-lined platter near your workspace. Arrange the bacon strips in a single layer on a large, rimmed baking sheet and transfer to the oven (no need to wait for it to be preheated). Roast the bacon for 18 to 20 minutes, or until it is crisped. Transfer to the prepared platter, let cool for about 2 minutes and then coarsely chop. While the bacon cooks, preheat a waffle maker to medium-high heat. (If making more than a couple of waffles at a time, once the bacon is cooked, reduce the oven temperature to 200 degrees so you can keep the waffles warm as you prepare them.) In a large bowl, combine the hash browns, cheese, eggs, flour, scallions, butter, baking powder, salt, if using, and pepper. Add the bacon and stir until well combined. For a Belgian waffle maker, add about 1 1/2 cups of the batter (you may need less, depending on your waffle maker) and cook according to the manufacturer’s instructions for about 5 minutes, or until golden and crisp. Transfer the waffle to a plate or place on a baking sheet in the warm oven. Repeat with the remaining batter. Serve waffles with a dollop of sour cream and sprinkle with sliced scallions or chives, if desired.