Bring a large pot of salted water to a boil. Add the macaroni and cook according to the package instructions until al dente, usually about 7-8 minutes. Drain the pasta and set aside.
While the pasta is cooking, steam or blanch the broccoli florets. To blanch, bring a small pot of water to boil, add the broccoli, and cook for 2-3 minutes until bright green and slightly tender. Immediately transfer to an ice bath to stop cooking and preserve color. Drain and set aside.
In the same pot or a large saucepan, melt the butter over medium heat. Once melted, whisk in the flour and cook for about 1-2 minutes, stirring constantly, to form a roux. It should be bubbling but not browned.
Slowly add the milk while whisking continuously to prevent lumps. Continue cooking and whisking until the sauce thickens, about 3-5 minutes.
Reduce the heat to low, then stir in Dijon mustard, nutmeg, garlic powder, salt, and black pepper. Gradually add the shredded cheese, stirring until fully melted and smooth.
Add the cooked macaroni and broccoli back into the cheese sauce. Stir gently to combine, ensuring every piece of pasta and broccoli is coated in cheesy goodness.
Serve your Broccoli Mac and Cheese hot, garnished with extra cheese or a sprinkle of black pepper if desired. It pairs beautifully with a fresh green salad or crispy bread for a complete meal.