01 - Grate the starchy potatoes using the large holes of a box grater. Transfer to a clean tea towel and squeeze out as much liquid as possible; discard the liquid.
02 - Boil the waxy potatoes in salted water until just tender, 10 to 12 minutes. Drain thoroughly and mash until completely smooth.
03 - In a large bowl, mix together the grated raw potato, mashed potato, flour, baking powder, salt, and pepper until well incorporated.
04 - Stir in the melted butter and milk until a thick, cohesive batter forms.
05 - Place a large non-stick skillet or griddle over medium heat and add a knob of butter to coat the surface.
06 - Drop large spoonfuls of batter, about ¼ cup each, onto the heated skillet, flattening each slightly with the back of the spoon.
07 - Cook 3 to 4 minutes per side, until golden brown and crisp. Work in batches as needed, adding more butter between batches to prevent sticking.
08 - Transfer the hot boxty to plates and top with a dollop of sour cream. Garnish with chopped chives or scallions if desired.