Preheat the oven to 350 F. Cook pasta according to package directions, drain. In a large saucepan, melt butter. Cook flour and cook for a minute to brown. Add in milk, salt, pepper, and 1 1/2 cups shredded cheddar cheese.
Cook the mixture, while stirring constantly until the cheese is fully melted and the sauce is thickened. Mix the cheese sauce with the pasta.
Evenly spread the pasta into a greased baking dish. Top with the remaining 1/2 cup shredded cheddar cheese. Top with the crushed Doritos chips. Bake for 20-25 minutes.