In an electric stand blender or food processor blend egg yolks, lemon juice, Dijon, salt, cayenne, and freshly ground black pepper.
Process on high speed until slightly thickened and lemon colored.
Melt the butter in a small pot or in the microwave in a glass measuring cup.
Turn blender to low speed; add butter to egg yolk mixture slowly in a thin steady stream.
Turn blender to the high speed setting and process until thickened, around 30-40 seconds or until desired consistency is reached.
On the stovetop pour into a double boiler or a glass bowl over simmering hot water (not boiling) to keep warm until serving. (Serve within 30 minutes)
Notes
Utilize a Double Boiler to Keep Warm - After making the sauce in the blender, it's simple to keep hollandaise sauce warm in a double boiler until serving. This is especially helpful for holiday brunch meals and special occasions when you need to do a bit of the prep work in advance.
Drizzle Very Hot Butter Slowly - It's important to drizzle the melted butter into the egg mixture in a thin steady stream.
Pepper - You can use white pepper in place of black pepper and smoked paprika in place of cayenne pepper.