Béarnaise sauce is a piquant child of hollandaise, one of the so-called mother sauces of French cuisine. It is simply an emulsification — egg yolks and butter cut through with vinegar flavored with tarragon and shallots, with a bite of black pepper. Think of it as a loose mayonnaise, requiring only plenty of whisking and a careful hand with the heat to master. You don’t need the clarified butter many recipes call for — a good unsalted butter, melted, works just fine. Apply the sauce to steaks or burgers, asparagus or salmon. The sauce’s richness improves virtually everything it touches.