In a small bowl, combine the flour, baking powder and salt; set aside.
In a large mixing bowl, combine the mashed potatoes with the grated raw potato, then add the flour and mix well.
Slowly add the buttermilk and stir gently (do not over mix). The mixture should be like a very firm, thick batter; almost like a dough, so add more buttermilk if needed.
Heat a griddle or nonstick frying pan over medium-high heat, and add a pat of butter, just before scooping out some of the boxty batter onto the pan.
Flatten and shape into a nice, round pancake shape and fry until golden brown on the bottom.
Turn and continue to cook until golden brown on top too, turning the heat down if they are browning too quickly (remember there are raw potatoes which need to cook).
Continue to add a little butter and fry the boxty until all the batter is finished. Serve hot!
