A terrific sauce can take a dish from good to great. In this episode, Martha goes over some classics, starting with Béarnaise Sauce, a derivative of Hollandaise. Next she makes Kansas City Barbecue Sauce, which is tomato-based with a sweet-tart flavor. Tartar Sauce– Martha's variation includes capers, shallots, and cornichons– and a delicious Cocktail Sauce are both beloved staples. Then Martha reveals us how to make Salsa Verde, a perfect accompaniment to grilled meats, poultry, seafood, and…