Bring a large pot of salted water to a boil. Cook elbow macaroni until al dente, 1-2 minutes less than package directions. Drain thoroughly and set aside without rinsing.

Cut chicken breasts into uniform 1-inch cubes and pat dry. Season with salt and pepper. Heat a large skillet over medium heat with 1 tablespoon butter. Add chicken in a single layer and sauté for 5-7 minutes until golden and cooked through to 165°F. Transfer to a plate.

In the same skillet, melt the remaining 3 tablespoons butter over medium heat. Sprinkle in the flour and whisk constantly for 1-2 minutes until smooth and lightly golden to create a roux.

Slowly pour in the milk while whisking constantly, starting with about half a cup then adding the rest in a steady stream. Continue whisking and cooking for 4-5 minutes until the sauce thickens and coats the back of a spoon.

Reduce heat to low. Add garlic powder, salt, and pepper. Gradually stir in the shredded sharp cheddar in three or four additions, stirring each batch until completely melted before adding more. Keep heat low to prevent graininess.

Add the drained macaroni and cooked chicken to the cheese sauce. Fold together gently until everything is evenly coated.

For stovetop version: Serve immediately while hot, garnished with fresh parsley. For baked version: Transfer to a greased 9x13-inch baking dish, sprinkle with breadcrumbs, and bake at 350°F for 20 minutes until golden brown and bubbling. Let rest 5 minutes before serving.