@UnaSpenser then, add more milk, with very small quantities at first, until your sauce is of the desired consistency. It can easily take half a liter for that nut of butter. It works better if the milk is room temperature, because it doesn't solidify the melted butter/flour mixture. If you add the milk too fast, hard balls of butter and flour will remain, and they are called "grumeaux" and will ruin the creamy aspect.