Separate egg yolks and add to tall container for immersion blending.
Add lemon juice, mustard and salt.
Melt butter in glass measuring cup in microwave until just warm and melted.
Place immersion blender into bottom of container and turn on to whip the egg yolk mixture until just blended (a few seconds). With the blender still running, gradually pour in the melted butter. Move the blender up and down a few times to incorporate all of the liquid butter. When you have a smooth creamy yellow thick sauce, you are done! Don't overbeat it.
Spoon the sauce over your Eggs Benedict or whatever else needs this awesomeness added to it!
One quarter of this recipe (a generous sauce amount on one serving of Eggs Benedict) is 245 kcals, of which 95% is fat calories. The fat breakdown is 68% saturated fat and 28% g monounsaturated.