2 Tbsp. butter
2 Tbsp. flour
1 cup milk
In a heavy saucepan, melt the butter. Add the flour, mixing well, and cook over low heat for one minute. Pour in the milk in a thin steady stream, stirring the whole time, and mix carefully to get any lumps out. Cook over low heat until thick, stirring occasionally.
If, like me, you have a tendency to burn milk-based things on the stove, you can use the same recipe in the microwave. Be sure to use a big container -- I use a four cup glass measure -- and to do things at one minute intervals. Melt the butter, add the flour and cook for 1 minute on high, add the milk and cook for 1 minute at a time, on high, stirring in between.