Add the butter bit by bit, whisking all the time. As soon as one piece melts, add the next. The mixture will gradually thicken, but if it shows signs of becoming too thick or “scrambling” slightly, remove from the heat immediately and add a little cold water if necessary. Do not leave the pan or stop whisking until the sauce is made. Finally, add the lemon juice to taste. Pour into a bowl and keep warm over hot, but not boiling, water.