Place 2 large sweet potatoes in a pot and cover with cold water. Bring to a simmer over medium-high heat and cook until easily pierced with a knife, 25 to 35 minutes. Drain and let cool slightly, then cut into 1/2-inch-thick wedges.
Brush the sweet potatoes with 1 tablespoon olive oil and sprinkle with 1 1/2 teaspoons chili powder and a little salt. Grill over medium-high heat, turning once, until well marked, 2 to 5 minutes per side. Remove to a platter; drizzle with olive oil and sprinkle with flaky salt.