Clarify butter by letting it stand in a water bath. Carefully decant butterfat from settled buttermilk and chill to about 45 °C.
For the peppercorns, reduction, shallots, salt, white wine vinegar, white wine and water (1) as good as completely reduce: the flavors are thereby first leached, and then concentrated. Then dilute with water (2): the aggressive acidity of the reduction would otherwise impair the emulsifying effect of the egg yolk lecithins.
Whisk the diluted reduction together with egg yolk in an 80 °C water bath to a loose creamy consistency.
Outside the water bath, regularly whisk the clarified butter into the egg yolk mixture in a fine thread.
Season with salt, cayenne and juice of one lemon.
Press the sauce through a dry straining cloth.
Meanwhile, keep the sauce in a lukewarm place with the lid closed.
If the sauce curdles, beat an egg yolk with a little water in a bain-marie and stir in the curdled sauce drop by drop. As soon as this part starts to thicken, you can stir in the rest a little faster.
Tip: Instead of clarified butter, you can also use butter in most cases.