Bring the water to a boil in a saucepan over high heat.
Reduce to low heat and begin to add chunks of the butter - a little at a time - while constantly whisking so it emulsifies (combines). When it begins to emulsify, add more butter. It is very important to keep the temperature within the range of 160 - 189 degrees Fahrenheit to ensure the emulsion becomes permanent. It must by no means come to a boil.
When it has become a very thick butter sauce, remove the saucepan from the heat. Your Beurre monté should be used within an hour.