Slowly whisk in the milk until smooth. Simmer until slightly thickened, whisking occasionally so the bottom doesn't burn.
In a large saucepan, melt the butter over medium heat. Sprinkle the flour over and whisk in. Cook the roux for 1-2 minutes to cook off the raw flour taste.
Bring a large pot of salted water to a boil. Cook the pasta to al dente, drain.