Preheat oven to 350. Cut a combination of sweet potatoes and white potatoes into large bite-size pieces (unpeeled). Toss in a bowl with olive oil and sea salt. Lay out on a sprayed jelly roll pan in single layer and roast for around an hour, turning once or twice with a spatula. Heat oven to 400, brush potatoes with a little butter, sprinkle with more salt and roast a little while longer until they turn golden.