To make the béchamel, melt the butter in a saucepan over a moderate heat, then add the flour, mixing well to incorporate. Cook the mixture until it begins to stick and forms a paste at the bottom of the pan. Gradually add the hot milk (removing the bay leaves), whisking all the time and switching to a wooden spoon as the mixture begins to thicken. Cook over a moderate heat for around 10 minutes, until smooth and thick. Remove from the heat and cover to prevent a skin forming.