set up a double boiler type cooking method with a small pot of simmering water and a bowl resting on top of the pot, but not touching the water
In the bowl whisk off the pot (no heat at this time) the egg yolks, water, and lemon juice until thick and pale
Set the bowl on the pot over the simmering water and continue to whisk making sure to get all the sides of the bowl
To moderate the heat, frequently move the bowl off the heat for a few seconds, and then back on. (If, by chance, the eggs seem to be cooking too fast, set the pan in the bowl of cold water to cool the bottom, then continue).
As they cook, the eggs will become frothy and increase in volume, and then thicken. When you can see the pan bottom through the streaks of the whisk and the eggs are thick and smooth, remove from the heat.
By spoonfuls, add the soft butter, whisking constantly to incorporate each addition. As the emulsion forms, you may add the butter in slightly larger amounts, always whisking until fully absorbed. Continue incorporating butter until the sauce has thickened to the consistency you want.
Season lightly with salt, pepper, and a dash of cayenne pepper, whisking in well. Taste and adjust the seasoning, adding droplets of lemon juice if needed. Serve lukewarm