Cook potatoes: Bring a pot of water to the boil, add 1 tbsp salt. Cook potatoes until soft - small ones should take around 20 to 25 minutes, medium ones might take 30 minutes. It's ok if the skin splits. Alternatively, steam or microwave them.
Preheat oven to 200°C/390°F (180°C fan).
Steam dry: Drain the potatoes and let them dry in the colander for 5 minutes or so.
Smash! Place on the tray then use a large fork or potato masher to squish them, keeping them in one piece. Thin = crisper. Thicker = fluffier insides. (Both good!) More nubbly surface = better crunch!
Steam dry again: Leave on the tray to steam dry for 5 minutes or so - makes them crispier!
Drizzle: Drizzle with butter, then olive oil. Sprinkle with salt and pepper.
Bake: Bake for 45 minutes (small potatoes) to 55 minutes (medium) or until deep golden and crispy. Do not flip!
Serve hot, sprinkled with parsley if desired.