Melt butter in a pan over medium heat or in a microwave oven.
Put egg yolks, 1 tablespoon lemon juice, 1 tablespoon water, and mustard in a blender or food processor.
Whirl on high speed to blend, then add butter in a slow, steady stream.
Add remaining lemon juice, cayenne, and salt to taste; whirl just to blend. Use at once or store in a thermos up to 30 minutes.
Nutritional analysis per tablespoon.