Separate the egg yolks from the whites and place the yolks in a heatproof bowl.
Add lemon juice to the egg yolks and whisk until slightly thickened.
Place the bowl over simmering water, whisking continuously for about 2-3 minutes.
Drizzle in the melted butter gradually while whisking vigorously to emulsify.
Once the sauce thickens and becomes creamy, remove it from heat.
Season with salt and cayenne pepper to taste.
If too thick, whisk in a teaspoon of warm water to adjust the consistency.
Serve immediately or keep warm until ready to use.