In a large bowl, whisk egg, buttermilk, 2 tsp. salt, and a few grinds of pepper. Add chicken and toss to coat. Cover bowl and refrigerate at least 2 hours and up to overnight.
In a large shallow bowl, whisk flour, cornstarch, paprika, garlic powder, onion powder, a few grinds of pepper, and remaining 3 tsp. salt.
Into a large, heavy pot fitted with a deep-fry or candy thermometer, pour oil to a depth of 2". Heat over medium-high heat until thermometer registers 350°.
Working one at a time, remove chicken from marinade, letting excess drip off. Toss in flour mixture until coated. Dip into marinade again, then toss in flour mixture once more. Arrange on a baking sheet.
Working in batches and adjusting heat to maintain oil temperature, fry chicken, turning occasionally, until deeply golden and an instant-read thermometer inserted into thickest part registers 165°, 3 to 4 minutes. Transfer to a wire rack.
In a medium bowl, stir mayonnaise, ketchup, Worcestershire sauce, garlic powder, pepper, salt, and onion powder to combine.
Arrange chicken on a platter. Serve with sauce and fries alongside.
Make Ahead: Sauce can be made 3 days ahead. Store in an airtight container and refrigerate.
