Cook macoroni until tender per box instructions. Drain well but do not run under cold water. Place in a casserole dish.
When you add the macaroni to your water is about the time you want to start your cheese sauce. It's better to start it too late than too early so it's not sitting getting hard or burning.
Cube up both packages of the cheese into a sauce pan and pour the cup of milk in as well. Turn stove on low. When it starts to melt use a whisk and slowly keep stirring it so neither the milk nor the cheese burns on the bottom and to mix the two together well. When fully melted pour this mixture on top of your macaroni and stir it in with the noodles well.
Take the 15 pieces of Tostitos and place them in a small ziplock bag and run over them a few times with a rolling pin. You want small pieces but not dust. Sprinkle this as a topping on the mac and cheese but don't stir it in.
Bake at 400 degrees F for about 10-15 minutes until the topping is browned. Remove from oven carefully and let stand for at least 5 mins so it will quit bubbling and is not too hot to eat.
Serve and enjoy.