Add the butter to a microwave-safe one-cup or larger measuring cup. Cover with plastic wrap and microwave on High 30 to 45 seconds, or until completely melted. Remove from the microwave and whisk in the lemon juice and water. Set aside to cool to lukewarm.

Meanwhile, separate the eggs, reserving the whites for another use. Add the yolks to a small bowl and whisk in the salt. When it has cooled, whisk a tablespoon or two of the butter mixture into the yolks to temper them. 

Pour the egg mixture into the butter mixture and whisk to blend. Cover loosely and microwave on Medium (see note) in 15-second increments, whisking well after every 15 seconds. After about three 15-second increments (or 45 seconds total), the egg yolks should begin to thicken around the edges of the container. 

The sauce will continue to thicken as it stands. If a thicker sauce is desired, microwave on Medium about 5 seconds more; if a thinner sauce is desired, whisk in warm tap water, a teaspoon at a time, until you reach the desired consistency.