How to Make It
In a microwave-safe 2-cup glass measure in a microwave oven at full power (100%), or in a 1- to 2-quart pan over medium-high heat, melt butter until bubbling.
Meanwhile, in the bottom of a double boiler or in a 2- to 3-quart pan, bring 1 inch water to a simmer over high heat; adjust heat to maintain simmer.
Put yolks, lemon juice, and mustard in the top of the double boiler or in a round-bottomed bowl (about 2 qt.). Nest the container over simmering water and whisk to blend.
Whisking, add butter in a slow, steady stream, taking about 1 1/2 minutes. Whisk until sauce reaches 140°; adjust heat to maintain temperature (remove from simmering water if necessary) and continue to whisk for 3 minutes. Immediately remove from simmering water. Serve hollandaise hot, warm, or at room temperature.