Peel the russet potatoes and cut them into quarters. Place the potatoes in a large pot and cover with water. Add a pinch of salt.
Bring the water to a boil and cook the potatoes until tender, about 15-20 minutes. Check by piercing them with a fork.
Drain the potatoes and return them to the pot. Mash them using a potato masher or ricer until smooth.
In a mixing bowl, combine the mashed potatoes with milk, butter, garlic powder, onion powder, salt, and black pepper. Mix until well combined and creamy.
Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a separate bowl, mix the all-purpose flour and cornstarch together.
Using a fork or spatula, scoop portions of the mashed potato mixture and roll them into fries. Dredge each fry in the flour-cornstarch mixture to coat thoroughly.
Place the coated fries on the prepared baking sheet, making sure they are not touching each other.
Lightly drizzle the tops of the fries with vegetable oil to promote crispiness.
Bake in the preheated oven for 25-30 minutes, turning halfway, until golden brown and crispy.