Béarnaise is the steak-lover's hollandaise — a warm butter emulsion built on a tarragon-and-shallot-infused vinegar reduction rather than plain lemon, giving it an aromatic complexity that makes it the undisputed partner for grilled beef. The reduction is the soul of the sauce: shallots in fine brunoise, cracked white peppercorns, and a generous handful of tarragon stems (save the leaves) are simmered in white wine vinegar and dry white wine until reduced to two tablespoons of fragrant, thick liquid. This is strained, cooled slightly, and whisked with egg yolks over a bain-marie at 55-62°C. The technique from this point mirrors hollandaise exactly: clarified butter in a thin stream, whisking constantly, until the sauce reaches a thick, spoonable consistency. The reserved tarragon leaves, cut to a fine chiffonade, are folded in at the end with a pinch of chervil. The finished béarnaise should hold its shape on a spoon, taste of tarragon and vinegar with rich butter in the background, and carry a gentle warmth from the white pepper. The reduction must be strong enough to assert itself through the butter — if the sauce tastes primarily of butter, the reduction was too dilute. Béarnaise tolerates no reheating — it is made à la minute and served immediately. The bain-marie temperature is critical: below 55°C the yolks will not thicken; above 65°C they scramble into an irreversible curdle.
Reduction is the soul — tarragon, shallot, vinegar, wine reduced to 2 tablespoons. Bain-marie at 55-62°C — below 55°C yolks won't thicken, above 65°C they scramble. Clarified butter in thin stream, whisking constantly. Fresh tarragon chiffonade folded in at the end. Cannot be reheated — strictly à la minute.
Double the tarragon in the reduction and strain it out — this gives a deeper tarragon infusion than the standard recipe. A tablespoon of tarragon vinegar (homemade, with fresh tarragon steeped in white wine vinegar for 2 weeks) used in place of plain white wine vinegar amplifies the herb character throughout. If the sauce begins to separate, whisk in a teaspoon of cold water and remove from heat immediately — the thermal shock can rescue a marginal emulsion.
Weak reduction — the tarragon flavour cannot assert itself through all that butter. Overheating the bain-marie — scrambled eggs in butter is not béarnaise. Using dried tarragon in the final addition — the flavour is completely different from fresh. Holding too long on the bain-marie — the sauce thins and eventually breaks after 20-30 minutes.
Escoffier, Le Guide Culinaire; Larousse Gastronomique