Whisk the egg and buttermilk together in a shallow bowl. Mix the 1 cup coating flour, garlic powder, onion powder, smoked paprika, cayenne, 1/2 tsp salt, and 1/4 tsp black pepper in a separate wide bowl. Dip each chicken chunk in the egg mixture, press firmly into the seasoned flour, and rest on a plate for 5 minutes.

Heat the vegetable oil in a large skillet over medium-high until it reaches 175°C (350°F), about 4 minutes. Fry the chicken in a single layer for 4 to 5 minutes per side until deep golden and the internal temperature reaches 74°C (165°F). Transfer to a wire rack or paper towel-lined plate.

Pour off most of the oil, leaving about 2 tablespoons in the pan. Reduce to medium heat, add the butter, then stir in the 2 tbsp gravy flour for 1 minute. Whisk in the chicken stock gradually, add the remaining 1/2 tsp salt and 1/4 tsp black pepper, and stir for 3 to 4 minutes until the gravy coats the back of a spoon.

Heat the corn in a small saucepan over medium heat for 2 to 3 minutes. Warm the mashed potatoes in the microwave in 60-second bursts until steaming.

Divide the rice among 4 bowls. Spoon the mashed potatoes over the rice, add the corn, top with the fried chicken, ladle the gravy over everything, and finish with the shredded cheddar cheese.