01 - Wrap grated raw potatoes in a clean kitchen towel and squeeze firmly to extract excess moisture until potatoes feel dry to the touch.
02 - Combine grated raw potatoes, mashed cooked potatoes, flour, baking powder, salt, and pepper in a large bowl. Gradually stir in milk until a thick, spoonable batter forms.
03 - Heat 1 tbsp butter and 1 tbsp oil in a large non-stick skillet over medium heat until butter melts and mixture shimmers.
04 - Drop heaped spoonfuls of batter into the hot pan, using the back of the spoon to gently flatten each mound into a pancake approximately 1/2 inch thick.
05 - Cook for 3 to 4 minutes per side until pancakes develop a deep golden-brown crust and feel firm when tapped. Add remaining butter and oil as needed between batches.
06 - Transfer cooked boxty to paper towels to drain excess oil. Serve immediately with sour cream and fresh chives sprinkled on top.