Place a rack in center of oven; preheat to 450° and oil a rimmed baking sheet
Place potatoes in a large pot and cover with water. Add 1 cup salt and bring to a boil. Immediately reduce heat to medium-low, bring to a simmer, and cook until potatoes are just cooked through and pierced easily with a fork, about 15 minutes (time will vary depending on the size of your potatoes; be careful not to overcook as they’ll fall apart when smashed).
Drain and allow to cool until you're able to handle them. Transfer potatoes to the baking sheet. Let cool slightly.
Use the palm of your hand to smash the potatoes each one (alternately, use another rimmed baking sheet on top, or the bottom of a mug or measuring cup)
Drizzle ⅓ cup oil over potatoes, then toss to coat; season lightly with salt and black pepper. Roast potatoes until they're crispy and golden brown (~ 35–40 minutes)