Mix together the flour, salt and baking powder. Add this mixture to a mixing bowl containing the mashed potatoes. Stir together (mixture will be a bit crumbly).
Mix the beaten egg with the milk and then add this into the potato mixture. Mix to a soft dough. Turn on to a floured surface and knead 4 or 5 times. (To knead, sprinkle the top of the dough with flour and then lift the bottom half of the dough onto the top half, while pushing in with the heels of your hands).
Roll or pat out to a 3/4 inch thickness and cut into rounds using a 2 1/2 inch floured cutter or a juice glass. Mix the leftover pieces together and roll out again, until all the dough has been used up.
Place on an ungreased cookie sheet. Bake in a 400 F (200 C) oven for 20 minutes or until lightly browned.