In an electric stand blender combine egg yolks, lemon juice, Dijon, salt, cayenne, and freshly ground black pepper.
Process on high speed until slightly thickened and lemon colored.
Turn blender to low speed; add butter to yolk mixture in a slow steady stream.
Turn blender to the high speed setting and process until thickened, around 30-40 seconds
On the stovetop pour into a double boiler or a glass bowl over hot, not simmering, water to keep warm until serving.