For a pouring sauce, use 15 g each butter and flour to 300 ml milk, for a coating sauce, 22 g each butter and flour. Cook flour just long enough to burst the starch grains and avoid a raw taste. To prevent lumps and give an even colour, stir roux constantly over the entire base of pan.
Add flour to melted butter. Stir with a wooden spoon over a low heat to create a white roux, 1-2 minutes.
Remove pan from heat. Gradually add hot milk, beating constantly to blend it with the roux.
Bring to the boil, stirring constantly. Lower the heat and simmer until the sauce reaches the desired thickness.