Preheat oven to 400 degrees F. Line a baking sheet with parchment paper and lightly add cooking spray. Set aside.
In a bowl, toss sweet potato fries with olive oil, chili powder, salt and pepper. Place on baking sheet. Bake until sweet potatoes are tender, about 25-35 minutes.
Meanwhile, make the creme fraiche. In a skillet, cook scallions over high heat for 5-6 minutes, until semi-blackened, then chop coarsely. In a bowl, mix together charred scallions, sour cream, buttermilk, salt and pepper until well combined.
Serve Japanese sweet potato fries with creme fraiche. Enjoy!