Place high-starch potatoes in a large pot and fill with water until submerged. Add salt and bring to a boil. Cook for 20 to 30 minutes until fork-tender.

Preheat the oven to 200°C (390°F) or 180°C (350°F) for fan-assisted ovens.

Drain the potatoes and let them sit in a colander for 5 minutes to steam dry.

Transfer the potatoes to a baking tray lined with parchment. Smash each potato gently with a fork or cup until cracked and flattened.

Let the smashed potatoes rest for another 5 minutes to evaporate moisture.

Drizzle melted butter and olive oil over each potato, then sprinkle with salt and black pepper.

Bake in the preheated oven for 45 to 55 minutes until golden brown and crispy.

Remove from the oven, let cool briefly, and serve hot, optionally garnished with chopped parsley.