In a bowl, pour buttermilk over grated potato. Sift together flour, baking powder and salt. Beat egg in a mixing bowl and add sifted dry ingredients, grated potato and mashed potato mixture. Mix until a thick, pancake-like batter forms (add more buttermilk or sifted flour if needed). Melt butter on a griddle over medium heat. Drop spoonfuls of batter onto griddle, making each about six inches in diameter. Brown on both sides.Garnish with sour cream and green onions if desired.