In a high speed blender, blend the yolks and the lemon juice on medium low speed until just combined.
Transfer the hot melted butter to a liquid measuring cup. With the blender running, remove the fill cap in the lid and pour the butter very slowly in a steady stream into the egg mixture. Blend until pale yellow and slightly thickened.
Remove the sauce from the blender and use immediately or set aside in a bowl, covered with plastic wrap, pressing it down until it touches the sauce. When ready to serve, stir in 1 or 2 tablespoons of hot water and stir until desired consistency.
Notes
Hollandaise sauce will thicken as it cools, when you want to reheat, DO NOT use the microwave. Heat the sauce in a small saucepan, over low heat with 2 more tablespoons of melted butter, stirring constantly for 1-2 minutes. Alternatively, add the sauce to a double boiler and allow the steam to reheat it while stirring constantly. If the sauce gets too much heat, the eggs will cook and the consistency of the sauce will be uneven.
Pouring the hot melted butter in very slowly ensures that the sauce will thicken. If poured too quickly, the sauce consistency may be thin.