In another saucepan melt the butter over a medium heat, whisk in the flour and keep whisking until you have a pale golden ‘roux’ or grainy looking paste. Return this saucepan to a medium heat and ladle the hot milk into the roux, a cup at a time, whisking all the time and completely incorporating each cup before adding the next one. After all the milk has been added, continue to whisk until the sauce thickens and bubbles gently (about 2 minutes).