Soft, silky, and light-textured, this tangy sauce is quick to assemble and surprisingly versatile. Without olive oil, eggs, or cream, this un-mayonnaise is nevertheless creamy and rich. Spoon it over poached seafood, or fold into meat, fish, or cooked vegetable salads. Sharpen it with hot pepper sauce to serve as a dip for crudités. Thin with additional buttermilk to sauce cooked vegetables, such as asparagus or summer squash. A fully ripe, oily avocado variety is a must.