Using a small pan on low heat, add egg yolks, lemon juice, and water and whisk to combine.
Continue whisking until mixture begins to thicken, turn slightly frothy, and you can see the bottom of the pan through the streaks from your whisk. If at any point the egg curdles, turn down the heat and start over.
Turn off heat and add chunks of butter, whisking to incorporate before adding more. Continue adding butter till you reach your desired consistency.,
Season with optional cayenne, salt, and pepper to taste.
Serve immediately. If the sauce sits, it will thicken. It can be thinned out again by adding a few dashes of water.