Preheat your oven to 350°F (175°C). Grease a pie dish with butter or non-stick spray.
Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a saucepan over medium heat, melt 2 tablespoons of butter. Whisk in the flour to make a roux, cooking for about 2 minutes until golden brown.
Slowly pour in the milk while whisking continuously to avoid lumps. Cook until the mixture thickens, about 3-5 minutes.
Stir in the shredded cheddar cheese until melted and smooth. Add Dijon mustard, salt, and pepper to taste.
Combine the cooked macaroni and crumbled bacon with the cheese sauce, ensuring everything is evenly coated.
Transfer the mac and cheese mixture into the prepared pie dish, spreading it out evenly.
In a small bowl, mix together breadcrumbs and melted butter. Sprinkle this mixture over the top of the mac and cheese.
Bake in the preheated oven for 25-30 minutes, or until the breadcrumbs are golden brown and the cheese is bubbly.
Let the mac n' cheese pie cool for a few minutes before slicing and serving.