Bring an inch of water to boil in small saucepan.
Mix egg yolks in glass bowl (that can fit into the saucepan without touching the water)
Place bowl on steaming water
Add ½ teaspoon water, ½ teaspoon sugar, pinch of salt, ½ Tbsp lemon juice
Whisk for about 30 seconds
Remove from heat and continue whisking until it gets a little lighter (about 30 seconds)
Put back on heat
Whisk until eggs thicken (1 or 2 minutes)
Add butter cubes a few at a time while continuing to whisk until they are all incorporated
If too thick, add more lemon juice (or warm vinegar, water, etc)
Salt to taste
Put into thermos until ready to use
