In a large bowl, whisk together the flour, garlic powder, onion powder, paprika, salt, black pepper, and cayenne pepper (if using).
In a separate bowl, whisk together the milk and egg.
Dip each chicken thigh in the milk mixture, then coat it in the flour mixture, making sure to shake off any excess flour.
Heat about 1 inch of vegetable oil in a large skillet over medium-high heat.
Once the oil is hot, carefully place the chicken thighs in the skillet, making sure not to overcrowd the pan.
Fry the chicken for about 6-8 minutes on each side, or until it is golden brown and crispy.
Use tongs to remove the chicken from the skillet and place it on a wire rack to cool.
Once all the chicken has been fried, serve it immediately with your favorite sides.