01 - Wrap grated raw potatoes in a clean kitchen towel and squeeze firmly to extract excess liquid until potatoes feel dry to the touch.
02 - Combine grated potatoes, mashed potatoes, flour, baking powder, salt, and pepper in a large bowl. Add beaten egg and milk, stirring until thick batter forms.
03 - Melt 1 tbsp butter with 1 tbsp oil in a large nonstick skillet over medium heat until butter foams slightly.
04 - Drop 1/4 cup batter portions into hot skillet, using spatula to flatten each into 3-inch rounds. Space pancakes evenly to prevent overcrowding.
05 - Fry for 3 to 4 minutes until edges appear set and bottom surface turns golden brown.
06 - Turn pancakes carefully and cook second side for 3 to 4 minutes until golden and crispy. Add remaining butter and oil as needed between batches.
07 - Transfer cooked boxty to paper towel-lined plate to drain excess oil. Serve immediately topped with sour cream and fresh chives.