Beat egg yolks, lemon juice, and salt in the top of a double boiler until a light lemon color.

Place insert over water at a low simmer. Add butter, a third at a time, while whisking constantly. You will see the sauce thicken.

Once all the butter has melted and blended in, continue to whisk over the water, still at a low simmer, for another minute or so, until sauce has thickened enough.

Serve at once.

Heat blender jar with hot water.

Melt butter in a small pan, and bring to a bubbling simmer (being careful not to allow to brown.)

Empty blender. Place egg yolks, salt, and lemon juice in blender jar, and blend a minute on Low.

Raise setting to High, and carefully and slowly pour the bubbling butter in a steady stream into the blender. when it is all in, blend another minute.

Stop the blender, stir down, cover again, and blend on High another minute or so to continue to thicken.