Béarnaise Sauce

Steps

1. 1
   Put the vinegar, wine, shallot, tarragon stalks, half the chopped tarragon leaves and the crushed pepper in a small pan. Simmer over medium heat for 8–10 minutes until reduced to about 2 tbsp, then strain, pressing on the solids, and let it cool slightly.
2. 2
   Melt the butter over low heat, skim off the white foam and keep it warm at about 60°C.
3. 3
   Put the yolks, the reduction and 1 tbsp water in a heatproof bowl over barely simmering water, without the bowl touching it. Whisk constantly for 3–5 minutes until thick and pale and the whisk leaves trails.
4. 4
   Never let the yolks pass 70°C — lift the bowl off the water as soon as it steams or starts to set at the edges. Overheated, they scramble.
5. 5
   Off the heat, whisk in the warm butter in a thin stream, drop by drop at first, until the sauce is thick and glossy. Leave the milky sediment behind in the pan.
6. 6
   Stir in the remaining tarragon and the chervil and season with salt. If it is too thick, loosen it with a few drops of warm water.
7. 7
   Serve at once, warm, with steak, grilled fish or asparagus. It will hold for up to 30 minutes over warm, not hot, water. If it splits, whisk a fresh yolk in a clean bowl and beat the broken sauce into it drop by drop.