To keep hollandaise sauce warm up to 2 hours, seal in a thermos. If making up to 4 days ahead, pour into a jar and chill airtight. To serve, let stand about 2 hours at room temperature, then whisk to soften.
1. Put yolks in a blender or food processor. Cut butter into chunks and put in a 1- to 2-quart pan over medium-high heat. Heat until melted and a little of the foam is beginning to brown (about 230°), 3 to 4 minutes.
2. Turn on blender or food processor and pour 1/4 cup boiling water into yolks, then add lemon juice and mustard. At once, pour hot butter into yolks in a steady stream, taking about 10 seconds (sauce will be thin). Sauce should be 160°. If it's not, pour into a metal bowl (about 3 qt.) or round-bottomed pan and nest over slightly simmering water in another pan. Whisk for 3 minutes.
3. Serve sauce hot or warm.
Nutritional analysis per tablespoon.