Put the sweet potatoes in a large bowl. Add the olive oil and toss to coat. Stir together the cornstarch, paprika, garlic powder, cayenne, 3/4 teaspoon salt and a few grinds of black pepper in a small bowl. Sprinkle the spice mixture over the sweet potatoes and toss to coat.
Arrange the sweet potatoes on a baking sheet in a single layer, leaving as much space between the fries as possible. Bake, turning once about three-quarters of the way through, until the edges are browned and crispy, 20 to 25 minutes. Cool 5 minutes before serving.
Serve the warm fries with the Ranch Dressing.
Add the buttermilk, mayonnaise, sour cream, vinegar, onion powder, garlic, 1 teaspoon salt, 1/4 teaspoon black pepper and hot sauce to a food processor. Pulse to combine until smooth. Add the parsley and dill and pulse to incorporate, 4 to 5 times. Season to taste with salt and pepper. Pour into a serving bowl.