Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
In a large bowl, toss cubed potatoes with olive oil, cornstarch, chili powder, paprika, cumin, garlic powder, salt, and pepper.
Spread the potatoes evenly on the baking sheet and roast for 30-35 minutes, flipping halfway through, until golden and crispy.
Warm the tortillas in a dry skillet or microwave until pliable.
Assemble the tacos by layering potatoes, lettuce, tomatoes, red onion, and a dollop of vegan sour cream in each tortilla.
Serve immediately and enjoy!