Bring 1 litre (4 cups) water to simmering point in a large saucepan or the base of a double boiler.
Place yolks, salt, 10 ml lemon juice and cold water in a small saucepan that will fit comfortably into the larger one, making a double boiler, or into the top of a double boiler.
Place smaller saucepan in larger one of boiling water, making sure the base doesn't touch the water.
Beat constantly with a balloon whisk or electric beater until eggs begin to thicken (almost immediately).
Remove smaller saucepan or top of double boiler from base (which remains on stove) and add first piece of butter, beating until it melts.
Add second piece, beating it in to thicken sauce.
Return to double boiler or larger saucepan and continue beating, over heat, until sauce begins to thicken. Remove immediately, still beating.
Continue as above until al butter has been used. Once thickened, place sauce over bottom half of double boiler on top of stove to heat through, beating rapidly. Taste and add a little extra salt and lemon juice, if needed. Serve immediately.
Makes 250 ml