In a heatproof bowl, whisk together the egg yolks and lemon juice until well combined.
Set the bowl over a saucepan of simmering water (make sure the bottom of the bowl is not touching the water) and whisk constantly until the mixture thickens and becomes light in color, about 2-3 minutes.
Gradually pour the clarified butter into the egg yolk mixture, whisking constantly, until the sauce is smooth and creamy.
Stir in the cayenne pepper (if using) and season with salt and pepper to taste.
Remove the bowl from the heat and serve the sauce immediately, or keep it warm in a heatproof bowl set over a pot of warm water until ready to serve.