Prepare the aioli: In medium bowl, whisk together egg yolk, Dijon mustard, and garlic until slightly thick and smooth. Slowly pour oil into egg yolk mixture - start with small drops, while whisking continuously with the other hand, then you can increase to a very slow thin stream - until the aioli is thick and emulsified. Slowly whisk in lemon juice until smooth. The aioli should be smooth, thick, and pale yellow in color. Season with salt and pepper to taste. Refrigerate until ready to serve.
Prepare the sweet potato fries: Using a large chef’s knife, carefully slice the potatoes into ¼-inch-wide matchsticks. Place in large mixing bowl and cover with cold water. Allow the potatoes to soak (to remove excess starch) for at least one hour or overnight.
Preheat the oven to 425°F (218°C) with rack in the center top position. Line a large sheet pan with heavy duty foil (dull side facing up) and lightly spray with cooking spray. Set aside. Drain potatoes in colander and pat dry with paper towels. Place potatoes in large, clean mixing bowl and toss with corn starch until they are very lightly coated. Drizzle with canola oil and toss.
In small bowl, combine paprika, cumin, chili powder, garlic powder, cayenne pepper, and kosher salt; sprinkle mixture over potatoes and toss until evenly coated.
Spread the sweet potato fries in an even thin layer (do not allow the fries to touch each other, if necessary use two sheet pans and rotate the pans halfway through cooking) and bake for 15 minutes. Flip fries halfway through and bake for an additional 12 to 15 minutes, or until caramelized.
Turn off oven heat, prop the oven door slightly, and allow fries to cool for 5 to 10 minutes; this will help crisp them up. Season fries with salt, garnish with chopped parsley (if using), and serve warm with the garlic aioli for dipping.