Preheat the oven to 220°C/425°F/gas 7. Sieve the flour, baking powder and salt into a bowl. Rub in the softened butter, then add the mashed potato and mix with a fork, adding just enough of the cold milk, 1 tablespoon at a time, to bind it to a soft dough.
Turn out on a floured surface and knead briefly, forming into a ball. Roll out to a thickness of 2 cm / ¾ inch and cut into triangles. Put on a lightly greased baking sheet and brush the tops with beaten egg yolk.
Bake for 10–12 minutes, when they should we well risen and golden brown. Serve while still warm.