Arrange a rack in the middle of the oven and heat to 350°F. Butter a 9×13-inch baking dish.
Bring a large pot of salted water to a boil. Add macaroni and cook 2 minutes less than al dente. Drain.
Shred 1 ½ pounds cheddar. Reserve 1 cup for topping.
Melt butter in the same pot over medium heat. Add flour and cook until golden, about 1 to 1½ minutes.
Gradually whisk in milk, ½ cup at a time, whisking out lumps. Bring to boil, then reduce to simmer for 5–7 minutes until thick.
Remove from heat. Add 5 cups cheese, salt, and pepper. Stir until melted.
Stir in the cooked macaroni to coat evenly.
Layer half the pasta in the dish. Sprinkle with half the reserved cheese. Repeat layers.
Sprinkle with paprika (optional). Bake for 10–15 minutes until bubbling and golden on top.
Let sit 5 minutes before serving.
