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PLACE EGG YOLK, MUSTARD, 1 teaspoon lemon juice, and salt in a mixing bowl and whisk together until mixture is smooth and thoroughly combined.
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To add the oil, start with one drop at a time and whisk constantly until the mixture begins to thicken; add remaining teaspoon lemon juice, then pour the oil in a very slow, steady stream and continue whisking until all the oil is incorporated and the sauce is thick and emulsified.
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3
Use immediately, or refrigerate in an airtight container for up to 1 week.
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4
Make reduction Combine wine, vinegar, shallot, and peppercorns in a small skillet over medium-high heat; cook until reduced to 1 tablespoon, 3 to 4 minutes.
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5
Add the boiling water and strain through a fine sieve into a heatproof nonreactive (stainless-steel or glass) bowl.
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6
Prepare bain marie (hot-water bath) Fill a medium saucepan with 2 inches of water and bring to a boil, then reduce heat so water is barely simmering.
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Heat egg yolks Add egg yolks to strained reduction and whisk, off the heat, until they become pale.
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Place bowl over the bain-marie.
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Whisking constantly, cook until the mixture is thick enough to hold a trail from the whisk and begins to hold its shape when drizzled from the whisk.
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Remove from heat.
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11
Wipe off any mixture that may have cooked onto the side of the pot with a damp paper towel to prevent any lumps from forming.
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Incorporate butter Whisking constantly, add butter 1 tablespoon at a time, whisking until each addition is incorporated completely before adding the next.
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When all the butter has been added, season with lemon juice, salt, and cayenne.
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The sauce should be thick but still able to drizzle from a spoon (and it should form a pool, not a mound).
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If it is too thick, thin it with a little water.
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16
Crack the side of a cold egg on a flat surfacenot on the rim of a bowl, which could shatter the shellthen use thumbs to pull it apart.
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Pour the yolk back and forth between the eggshell cups, letting the white run out into a bowl beneath.
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Then drop the yolk into a separate bowl.
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Transfer the white to a third bowl before repeating; if any yolk breaks and lands in the first bowl, you wont have to discard all the whites.
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20
To fix a broken emulsion (it will appear curdled), put 1 teaspoon of water (cold if its a warm emulsion, warm if its cold) in a bowl.
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Whisk sauce into water until its smooth and creamy again.
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22
Although traditionally made with melted clarified butter (page 88), softened butter emulsifies more readily with the egg yolks and produces a lighter texture.
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23
Do not overheat the egg yolks; temper them instead by mixing with a bit of boiling water before placing in the hot-water bath to keep them from scorching.
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Simmer over very low heat.
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If the egg mixture is heated too quickly, it turns grainy; if cooked too long over too high a temperature, it will scramble.
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Add butter gradually to allow the mixture to emulsify.
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Adding too quickly will cause the emulsion to break or separate, preventing the liquid and butter from combining.
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Adjust the finished sauce with water to thin, and add lemon juice, salt, and cayenne pepper to flavor.
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29
If not serving immediately, cover with plastic wrap, pressing it directly on the surface of the sauce to prevent a skin from forming, and set over a pot of water that has been brought to a simmer and then removed from heat, or in a warm spot on the stove for up to 1 hour.
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30
Alternatively, store in a clean thermos warmed with hot but not boiling water, holding it for 2 or 3 hours at most.
