In a large mixing bowl, combine cooled leftover mashed potatoes with all-purpose flour, salt, and melted butter. Mix until a soft dough begins to form, about 2-3 minutes.

Lightly flour a clean work surface and transfer your dough onto it. Gently knead for about 1-2 minutes until smooth and cohesive.

Using a sharp knife or a pizza cutter, cut the dough into quarters, forming your farls.

Preheat a non-stick skillet over medium heat for about 5 minutes.

Place each farl in the hot skillet, cooking for about 3-4 minutes on one side until golden brown and crisp.

Flip each farl and cook for another 3-4 minutes on the other side until golden-brown.

Remove from skillet and serve immediately, warm with butter or alongside eggs or jam.