Bearnaise sauce (French: Sauce béarnaise) is a sauce made of clarified butter and egg yolks flavoured with tarragon and shallots, with chervil and tarragon simmered in vinegar to make a reduction. "A Béarnaise sauce is simply an egg yolk, a shallot, a little tarragon vinegar, and butter, but it takes years of practice for the result to be perfect," wrote the restaurateur Fernand Point (1897-1955) in Ma Gastronomie. It is a traditional sauce for steak.
The sauce was likely first made by the chef Collinet, the inventor of puffed potatoes (pommes de terres soufflés) and served at the 1836 opening of "Le Pavillon Henri IV", a restaurant at Saint-Germain-en-Laye, not far from Paris. Evidence for this is reinforced by the fact that the restaurant was named for King Henry IV, a gourmet himself, who was born in the former province of Béarn.
Like Hollandaise sauce, Bearnaise sauce is an emulsion of butter in egg yolks. The difference is only in their flavouring: Bearnaise uses a reduction of vinegar and tarragon, while Hollandaise uses lemon juice. Such emulsions require some practice to prepare properly. The prime dangers are curdling the egg yolk mixture through excessive heat, and separation of the emulsion by rushing the addition of clarified butter. Beurre blanc sauce, yet another emulsion, was born from an error when making a Bearnaise sauce.