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 boil 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 1½ minutes. Whisk until sauce reaches 140°F adjust heat to maintain temperature (remove from simmering water if necessary) and continue whisk for 3 minutes.
Immediately remove from simmering water. Serve hollandaise hot, warm, or at room temperature.
This method produces a bacteria-safe sauce. However, if sauce gets too hot, it may separate. To bring back together, put 2 Tablespoons water in a blender or bowl. With motor running or while whisking, add the separated sauce in a slow, steady stream.
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