In a large pot of salted boiling water, cook the macaroni according to the directions on the label or until al dente. Drain.
In the same saucepan, melt the butter over medium heat. Whisk in the flour and cook, whisking constantly, for 2 minutes. Gradually whisk in the milk. Cook, whisking until the mixture thickens, about 4-5 minutes. Turn the heat to low, and add the cheddar and Velveeta in handfuls, whisking to melt. Stir until all the cheese is combined, and season with the mustard, cayenne, and 1/2 teaspoon salt, or to taste. Add the cooked pasta, and stir well to coat all of the pasta with the sauce. Enjoy immediately.