For the sauce
Heat a saucepan over medium heat and melt the butter. Add flour and stir continuously with a whisk so that the ingredients combine well and the flour changes colour slightly, about 5 minutes. Add the warm milk and continue to stir with the whisk until the ingredients have combined well. Bring to the boil, reduce the heat, season with salt and simmer for a further 20 minutes, stirring often so that the sauce doesn’t stick. Sieve using a chinois, taste for seasoning, cool rapidly and store in the fridge.