Preheat oven to 350 degrees (if baking).
Prepare macaroni according to directions on box, removing from the heat about 1 minute before recommended "al dente" cook time. If making GF noodles, check around 4 minutes. Drain noodles and set aside.
Melt butter in a large, oven safe skillet or dutch oven over medium heat.
Slowly add corn starch while stirring constantly with a whisk. Keep stirring for about 1 minute until the roux becomes bubbly.
Add milk, a little at a time, until sauce becomes thick (you want the consistency of gravy). Add more milk if necessary. If it gets too thin, you can mix a little corn starch with some milk and add it to the mixture to thicken.
Add dry mustard, salt and pepper to taste.
Add cheese (reserving a little if you want to add cheese to the top as a topping).
Add cooked macaroni to the sauce. Stir well to coat. If you are adding ingredients, stir them in now.
To bake, add reserved cheese to the top and place in pre-heated oven for 25-30 minutes or until cheese topping becomes bubbly (see notes above). If you want a creamier mac and cheese (not baked) reduce heat and cook on stove top until noodles are cooked through. You may need to add a little milk as needed.
Remove from oven and allow to cool for about 3-5 minutes before serving.