Notes: 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.
1. 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.
2. 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.
3. 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.
4. 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.