Bring a large pot of salted water to a boil. Add the elbow macaroni and cook until al dente, about 7-8 minutes. Drain the pasta and set aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Season the cubed chicken breasts with salt and pepper, then add to the skillet. Cook for 5-7 minutes, stirring occasionally, until the chicken is fully cooked and no longer pink.

In the same skillet, melt the remaining 2 tablespoons of butter. Once melted, whisk in the flour, stirring until smooth for about 1 minute. Gradually whisk in the whole milk, continuing to stir until the mixture thickens, approximately 4-5 minutes.

Lower the heat and gradually add the shredded cheddar cheese to the milk mixture, stirring until it melts completely and forms a velvety cheese sauce.

Gently fold in the cooked pasta and sautéed chicken. Mix everything until the pasta and chicken are well-coated in the cheese sauce.

For a crunchy topping, pour the mixture into an oven-safe dish. Sprinkle breadcrumbs on top and bake at 350°F for about 20 minutes until golden brown.