Steam the potatoes for 13-15 minutes, until tender. Tip into a clean, dry pan and heat until completely dried out, about 1 minute. Season and mash with the butter.
Mix in the plain flour, then, on a floured surface, roll out to about 0.5cm wide. Using a 9cm round cookie cutter, cut out 4 potato farls. Gather up and re-roll the scraps and cut out another 4 rounds.
Heat a lightly oiled griddle pan and carefully add 4 of the potato rounds. Cook for up to 2 minutes or until the underside starts to develop dark lines. Flip over and cook for another 1-2 minutes. Repeat with the remaining rounds.