In the container of a blender, combine the egg yolks, mustard, lemon juice and hot pepper sauce. Cover, and blend for about 5 seconds.
Place the butter in a glass measuring cup. Heat butter in the microwave for about 1 minute, or until completely melted and hot. Set the blender on high speed, and pour the butter into the egg yolk mixture in a thin stream. It should thicken almost immediately. You can serve the sauce right away as it is. OR -- Because I worry about the egg yolks not being cooked enough, I like to pour the sauce into a glass measuring cup which I then set in a pan of simmering water. I cook it slowly for about 5 minutes so it's good and hot and the yolks are completely cooked this way.