Place sweet potatoes in a large saucepan and add enough water to cover. Bring to a boil, reduce heat and simmer until barely tender, 6‑8 minutes. Drain well in colander and pat dry with paper towels.

Heat oil in a large cast-iron skillet over medium-high heat until just starting to simmer. Pat potato cubes dry again with paper towels; add to skillet. Let cook undisturbed 3 minutes; turn and continue cooking until crisped and browned on most sides, about 10 minutes. Sprinkle with salt, sugar, and cayenne, if using; serve immediately.