Wrap grated raw potatoes in a clean kitchen towel and squeeze firmly to extract excess liquid until potatoes feel dry to the touch.

In a large mixing bowl, combine grated potatoes, mashed potatoes, flour, baking powder, salt, and black pepper. Mix thoroughly until evenly distributed.

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.

Melt 1 tbsp butter with 1 tbsp vegetable oil in a large nonstick skillet over medium heat. Swirl to coat the pan evenly.

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.

Cook for 3 to 4 minutes until the bottom develops a deep golden-brown crust and the edges appear set.

Flip carefully and cook an additional 3 to 4 minutes until both sides are crisp and evenly browned. Adjust heat if browning too quickly.

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.

Arrange hot boxty on serving plates and top with generous dollops of sour cream. Garnish with fresh chives and serve immediately.