What this recipe shows:
Once the yolk-lemon juice mixture begins to thicken, it has reached a temperature high enough to kill salmonella.
Whisking in the melted butter over hot, not boiling, water off the heat prevents the yolks from scrambling.
Adding salt to the hollandaise after the ice cubes are added and the hot water has cooled prevents the yolks from scrambling.
4 large egg yolks
3 1/2 tablespoons fresh lemon juice (about 2 lemons)
1 tablespoon water
1/2 pound (2 sticks) unsalted butter, melted
1/4 teaspoon salt
1/8 teaspoon cayenne
Place the egg yolks, lemon juice, and water in the top of a double boiler or in a bowl resting over the top of a medium saucepan of simmering water. It is important that the top of the water be well below the upper part of the double boiler or the bottom of the bowl. Have the melted butter ready to drizzle in. Whisk constantly. Thesecond that the yolk mixture begins to thicken slightly, remove the top of the double boiler or the bowl from above the hot water and continue whisking. Turn off the heat. Add four ice cubes to cool the hot water a little. Put the pan or bowl of yolks back above the hot water. Whisk in the melted butter, drizzling it in very slowly. If at any time the sauce looks as if it is about to break, remove bowl and continue whisking to cool it down or whisk in 1 teaspoon cold water. With constant whisking, whisk in the salt and cayenne. When all the butter is incorporated, taste and add more salt or cayenne as needed.