Foolproof Béarnaise Sauce

"Rich, buttery, tarragon-flavored Béarnaise is the easiest elegant steak sauce to make at home. [Photographs: J. Kenji Lopez-Alt] For my money, the very best classic steak sauce you can make at home, a sauce that will wow your guests with its flavor and elegance, and—most importantly—a sauce that can be made start to finish in under half an hour, is béarnaise. The catch (there's always a catch) is that made with the classic technique, it's very easy to mess up. Here is a foolproof technique that uses hot butter and a hand blender for perfect results every time. Why this recipe works: * Blending hot butter into the vinegar and egg yolk mixture instead of whisking it over a double boiler makes for a foolproof and stable emulsion every time...."