Boil pasta until al dente. Pour into a 9x9 casserole dish. Set aside.
Preheat oven to 350°F.

Over low heat melt butter

Stir flour into melted butter. (I usually remove from the heat for a minute so it doesn't get too clumpy, the clumps will be gone by the end though)

Add milk to butter and flour and turn up heat to med-high.

Once mixture begins to bubble, add all the cheese and salt to it and stir until cheese is melted.
Remove from heat and pour into casserole dish carefully.

Bake, covered with tinfoil, for 25-30 minutes.
*This is also delicious topped with crackers, cheese itz, ritz, or a little extra cheese before baking.