Preheat oven to 400 degrees. Peel potatoes and halve lengthwise; slice crosswise 1/2 inch thick. On a baking sheet, toss potatoes with olive oil; season with coarse salt.
Transfer half the potatoes to a second baking sheet; cook both sheets until potatoes are tender, tossing occasionally, 25 to 30 minutes.
Sprinkle with butter, brown sugar, pecan pieces, and cayenne pepper, dividing evenly. Bake until sugar is caramelized and hard, about 10 minutes. Gently toss; serve immediately.