In a large bowl, combine the buttermilk mixture: buttermilk, garlic powder, onion powder, paprika, salt, and black pepper. Add the chicken tenderloins and make sure they are fully coated. Cover and refrigerate for at least 4 hours—this step is key for maximum flavor. You can also use a large sealable bag.
In a shallow dish, whisk together the flour mixture: all-purpose flour, garlic powder, onion powder, paprika, salt, and pepper.
Remove the chicken from the buttermilk marinade, but don’t let the excess drip off as this helps the coating stick. Dredge chicken in the flour mixture, pressing to ensure the coating sticks well. For a double coating, dip them back into the buttermilk mixture and again into the breading mixture.
Heat oil in a Dutch oven, cast-iron skillet, or high-sided pot over medium-high heat. Use a deep-fry thermometer to monitor the oil temperature, keeping it at 350°F.
Carefully place coated chicken tenders into the hot oil in a single layer. Fry each piece for 3 to 4 minutes per side, or until the second side is golden and the internal temperature reaches 165°F.
Transfer to a wire rack set over a paper towel-lined prepared baking sheet. Let the chicken rest for 5 minutes to drain excess oil and maintain crispiness.