Add egg mixture to top of double-boiler or steam-jacketed kettle. Cook over medium heat, stirring constantly, just until mixture becomes smooth and glossy. Remove from heat. Whisk in butter in a very slow, steady stream until incorporated. Return to heat and cook, whisking constantly, until temperature reaches 140°F. Shut off heat; cover and keep warm.

Béarnaise Sauce: Stir .2 oz. dried tarragon leaves and 2 oz. minced shallots into fully cooked Hollandaise.