In a stainless steel bowl set over a pot of simmering water over medium heat, whisk the egg yolks with the lemon juice, water, mustard, and parsley, together. Season with salt and cayenne. Whisk the mixture until pale yellow and slightly thick. Be careful not to let the bowl touch the water. Remove the bowl from the pot and whisking vigorously, add the butter, 1 teaspoon at a time, until all is incorporated. Fold in the caviar and serve warm.
Yield: about 1 1/2 cups