Add egg yolks, lemon juice, and salt to a tall, narrow cup that fits the head of an immersion blender.
Melt butter in a saucepan (or in the microwave!) until a thermometer registers the butter temperature at at least 200 degrees F.
Blend the egg mixture until egg yolks have slightly lightened in color, about 1 minute.
Stream hot butter into eggs a little at a time while you whisk, letting the hot butter cook the eggs and blending until hollandaise is light and fluffy and all butter has been incorporated. If the hollandaise gets too thick, blend in a few
teaspoons of water to loosen it before serving.