Mix together the flour, baking powder and salt in a bowl, rub in the butter using the fingertips until the mixture resembles breadcrumbs.
Add in the scallions now if you are using them.
Stir in the mashed potato and enough milk to make soft but not loose dough. Roll out onto a floured board into a round approx ½ inch thick and mark into quarters without cutting right through the potato farl.
Place on a greased baking tray and bake in the oven at 200 degrees for about 20 minutes.
Serve hot with plenty of butter.