Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
Wash and peel the potatoes, then cut them into thin fries (about 1/4 inch thick).
In a large bowl, toss the fries with vegetable oil, garlic powder, paprika, salt, black pepper, and cayenne pepper until well coated.
Spread the seasoned fries in a single layer on the prepared baking sheet.
Bake in the preheated oven for 20-25 minutes, flipping halfway through, until fries are golden and crispy.
While the fries are baking, prepare the nacho cheese sauce. In a saucepan over medium heat, combine milk and cornstarch, whisking constantly until it thickens (about 3-4 minutes).
Lower the heat and gradually stir in the cheddar cheese, garlic powder, onion powder, and jalapeños (if using) until smooth. Adjust salt to taste.
Once the fries are fully cooked, remove them from the oven and serve immediately with the warm nacho cheese sauce on the side for dipping.