In a blender, place the egg yolks, lemon juice, mustard, salt, and cayenne. Blend for 5 to 15 seconds, or until the mixture is frothy.

Place the butter in a small pot over medium-low heat. Cook, stirring, until melted. Transfer the melted butter to a glass liquid measuring cup so that it’s easy to pour.

With the blender running on low, slowly pour in the hot butter, blending until the sauce is emulsified.

The sauce should have a thick but pourable consistency. If it’s too thick, blend in hot water, 1 tablespoon at a time, to loosen it. Season to taste and serve immediately.