Prepare the pasta according to the directions on the box. Melt the butter over low heat in a medium sauce pan. Add in the flour to make a roux, whisking constantly. Cook about 1 minute before whisking in the milk. Continue whisking as the sauce thickens. Remove from the heat. Whisk in the seasonings (mustard, salt, hot sauce). Add in the cheeses using a spatula or wooden spoon until the sauce is smooth.