Melt butter in largish, heavy-bottomed saucepan over low-medium heat.
Add flour, stirring with whisk or spatula until it bubbles; cook, stirring constantly, 1 minute.
Do NOT let it brown. [If it browns, save it for brown gravy and start again.].
Remove from heat and mix in mustard, pepper, cayenne, and Worcestershire; blend well, then add milk APPROXIMATELY ONE TABLESPOON AT A TIME until it is soupy or gloppy. This method prevents lumps.
Return to medium heat and gradually add the rest of the milk, and the salt.
Stir or whisk constantly until it comes to a boil (a wooden spatula is a handy tool for this); simmer one minute, always stirring.
Remove from heat and add cheese; stir occasionally until all lumps of cheese disappear.
This takes 5-10 minutes, but you can return it to very low heat if necessary.
Note: Removing it from the heat is not absolutely necessary with Velveeta; but with natural cheese, it is best to melt it this way to avoid clumping and stringiness. Natural cheese will blend much better if it melts into the sauce off the heat, something about how the protein reacts to heat.
Meanwhile, slightly undercook cook elbows in salted water in large pot and drain and return to large pot.
Add sauce and mix gently with a lifting motion so the pasta isn't smashed.
Place in buttered or Pammed casserole dish; I like to use an 11x15 so I get lots of surface area for the crust, but if you don't like crusty mac&cheese use a smaller, deeper pan.
[At this point, or after mixing in large pot, you can refrigerate it for a couple of days if desired. Let it return to room temp or cook longer. You can also freeze, thaw, and spread in pan.] Top with bread crumbs if desired.
Bake at 350F for 1 hour or until it's bubbling well and top is browned the way you like it. Mmmm.