This is the classic base of a white sauce. To thicken any sauce and give it a velvety texture you need to put starch (e.g. flour) into it - but just dropping it in is likely to result in unwanted lumps. Traditionally, French chefs make a roux by heating equal weights of butter and flour until the moisture evaporates. The flour particles become coated in fat and are then easy to disperse in the sauce.