01 - Wrap the grated raw potatoes in a clean kitchen towel and squeeze firmly to extract as much liquid as possible, then transfer to a large mixing bowl.
02 - Add the mashed potatoes, flour, baking powder, salt, and black pepper to the bowl with the grated potatoes. Mix thoroughly until evenly distributed.
03 - Pour in the beaten egg and milk, then stir until a thick, cohesive batter forms. The mixture should hold its shape when dropped from a spoon.
04 - Melt 1 tbsp butter with 1 tbsp vegetable oil in a large nonstick skillet over medium heat, swirling to coat the bottom evenly.
05 - Drop 1/4 cup portions of batter into the hot skillet, spacing them apart. Flatten each mound gently with a spatula to form even rounds about 1/2 inch thick.
06 - Fry for 3 to 4 minutes until the bottom is deep golden brown and crispy. Watch for bubbles appearing on the surface as a sign the bottom is set.
07 - Turn each pancake carefully and cook the second side for 3 to 4 minutes until golden brown and cooked through. Adjust heat if browning too quickly.
08 - Transfer cooked pancakes to paper towels to drain excess oil. Add the remaining butter and oil to the skillet as needed, then repeat the cooking process with the remaining batter.
09 - Arrange hot boxty on a serving platter and top each portion with a generous dollop of sour cream. Garnish with fresh chives if desired.