2 tablespoons butter
2 tablespoons flour
1/4 tsp salt (optional)
1 cup milk

In a heavy saucepan, melt the butter. Sprinkle the flour over it and stir to incorporate. Cook over medium low heat for 2-3 minutes, stirring constantly. Slowly whisk in the milk.

Variations:

Cream of Chicken – If you want to make a cream of chicken flavored sauce, use half broth and half milk. If you have troubles getting this to be smooth, try heating the milk or broth before adding to the flour and butter. Bring to a simmer and cook over low heat, barely simmering, for a few minutes until thick.

Cheese sauce – To make a cheese sauce, add grated cheese and stir until melted.

Cream of Celery, Mushrooms or Onion – To make a cream of something, saute 1/4 cup or so finely chopped celery, mushrooms, or onion in the butter before adding the flour and use half broth and half milk.

Recipe found on Cheap Cooking.