If you intend to make a mac and cheese bake, preheat the oven to 200C or 400F. Measure out all your ingredients separately for smooth sailing - the milk, vegan butter, cornflour, nutritional yeast and the salt, plus the optional extras, if you're using them.
Heat a large pot of salted water over a high heat until it reaches boiling point. Add your pasta and cook as per packet instructions - only cook it half way if you intend to make a mac n' cheese bake. Drain and reserve a little starchy pasta water to add to the cheesy sauce.
While the pasta is cooking, place a medium pot over a medium heat.
Add the vegan butter to the pan and allow it to melt completely. Add the cornflour and whisk thoroughly to combine. Allow the paste to cook off for a minute or so before adding the milk. Whisk as you add the milk and intermittently for 10 or so minutes as the mixture thickens into a roux. You're looking for the consistency of a custard on the thinner side.
While the mixture is thickening, add the nutritional yeast and seasoning. If you're using them, add the miso paste, mustard and salt here, too. If you're using miso paste, add salt to taste as it is already quite salty.
Once the roux has almost completely thickened, stir in half the vegan cheese if you're using it. If not, take it off the heat. From here you can serve immediately as a classic mac and cheese, using the remaining half of the cheese to sprinkle over the top.