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 - In a large mixing bowl, combine grated potatoes, mashed potatoes, flour, baking powder, salt, and black pepper. Mix thoroughly until evenly distributed.
03 - Add beaten egg and milk to the potato mixture. Stir until a thick, cohesive batter forms. The consistency should be thick enough to hold its shape when spooned.
04 - Melt 1 tbsp butter with 1 tbsp vegetable oil in a large nonstick skillet over medium heat. Swirl to coat the pan evenly.
05 - Drop 1/4 cup portions of batter into the hot skillet, spacing 2 inches apart. Flatten each mound gently with the back of a spatula to form 1/2-inch thick rounds.
06 - Cook for 3 to 4 minutes until the bottom develops a deep golden-brown crust and the edges appear set.
07 - Flip carefully and cook an additional 3 to 4 minutes until both sides are crisp and evenly browned. Adjust heat if browning too quickly.
08 - Transfer cooked pancakes to paper towel-lined plates to drain excess oil. Keep warm in a 200°F oven while repeating with remaining batter, adding fresh butter and oil as needed.
09 - Arrange hot boxty on serving plates and top with generous dollops of sour cream. Garnish with fresh chives and serve immediately.