1. Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
2. In a large bowl, mix together the mashed potatoes, flour, baking powder, and salt until combined.
3. Fold in melted butter gently until incorporated.
4. Turn the dough onto a floured surface and knead lightly until smooth; shape into a round loaf.
5. Place on the prepared baking sheet, brush with remaining melted butter, and bake for about 30 minutes or until golden brown.
6. Let cool slightly before slicing and serving warm.
