Follow these seven steps for a soft, golden loaf. Read through them once first, since the dough needs two rises.

Step 1: Prepare the mashed potato

• Peel and chop one medium potato.
• Boil until fork-tender, then drain thoroughly and mash until smooth.
• Measure out 1 cup of mashed potato and allow it to cool until just warm or at room temperature.
• Use plain mashed potato without added milk, butter or seasoning for the most consistent results.

Step 2: Mix the dough

• In a large mixing bowl, combine the warm milk or water, sugar and instant yeast.
• Add the mashed potato, melted butter and egg if using. Stir until mostly combined.
• Add the flour and salt. Mix with a wooden spoon or sturdy spatula until no dry flour remains and a soft, slightly sticky dough forms.
• The dough should be soft and tacky rather than firm. If it seems dry, add extra liquid 1 tablespoon at a time. If it is extremely wet, add a little flour.

Step 3: Let the dough rise

• Cover the bowl with plastic wrap or a clean kitchen towel.
• Leave it in a warm, draft-free place for 60-90 minutes, or until roughly doubled in size.

Step 4: Shape the loaf

• Lightly grease a 9 x 5-inch (23 x 13 cm) loaf pan.
• Lightly flour your hands or use a spatula to gently fold the dough over itself a few times.
• Shape it into a loose log and place it seam-side down in the pan.
• Avoid adding too much extra flour, as the moisture from the potato helps make the bread fluffy.

Step 5: Let it rise again

• Cover the pan loosely and let the dough rise for another 30-60 minutes, until noticeably puffy and approaching the top of the pan.
• Meanwhile, preheat the oven to 350°F (175°C).

Step 6: Bake until golden

• Brush the top lightly with milk if desired.
• Bake for 35-40 minutes, until the loaf is golden brown and cooked through.
• If the top browns too quickly, loosely cover it with foil during the final 10 minutes.
• For a reliable doneness check, the center should reach approximately 190-200°F (88-93°C).

Step 7: Brush with butter and cool

• Remove the loaf from the oven and brush the top with melted butter for a soft, flavorful crust.
• Let it rest in the pan for 10 minutes, then transfer it to a wire rack.
• Allow it to cool for at least 45 minutes before slicing.