Heat the oven to 375°F.
Stir ⅔ cup mayonnaise, ½ cup finely grated Parmesan cheese, and 1 teaspoon garlic powder together in a small bowl.
Cut 2 large boneless, skinless chicken breasts in half horizontally to get 4 pieces total. Pat dry with paper towels. Season all over with 1 teaspoon kosher salt and ¼ teaspoon black pepper.
Place the chicken in a single layer in a 9x13-inch broiler-safe baking dish. Spread the mayonnaise mixture over the chicken in an even layer.
Bake until the chicken is cooked through and registers at least 165°F on an instant-read thermometer, 20 to 25 minutes.
Set the oven to broil. Broil until the top is golden-brown, 2 to 3 minutes.
