Making macaroni and cheese from a box is one of those quick, easy, hassle-free meals that still produces a satisfying and filling result. If you have run out of milk, however, you may have to choose whether the meal is worth the trip to the store. Fortunately, you do not need to make this decision. While the final result will be very slightly less creamy and rich, you can make an almost identical macaroni and cheese from a box without including milk.
Boil the pasta as directed on your box of macaroni. Drain as directed, but retain approximately 1/4 cup of the cooking water.
Place the cooked macaroni back into the pot or into whatever container you wish to mix it in. Add the butter. For a richer, creamer macaroni, add slightly more butter. This will help compensate for the lack of milk. Use 2 tbsp of butter as a rule of thumb, but if your box calls for more, use at least as much as the box dictates (if not slightly more).
Mix well to melt the butter, then add the cheese packet from the box and 1 to 2 tbsp of the retained cooking water. Mix well again to fully coat the pasta with the sauce mixture that should form. If the macaroni is not creamy or wet enough, add a little more butter and/or reserved pasta water and mix thoroughly again.
Season with any final ingredients you wish, mix one last time to blend, serve and enjoy.