What started out as a backyard barbecue for family on Sundays has emerged into the best place to gnaw on a slab of ribs in Dallas. Clarence Cohens just couldn't keep it all in the family, so eventually he began selling it just south of downtown in a two-room dive known as Baby Back Shak. His Memphis-style herb-rubbed ribs are so moist, tender and flavorful you don't even need any barbecue sauce to enhance. Since its recent discovery, the downtown lunch crowd swamps this place, which stays open until 5:45. No dinner done here. There are more than ribs, of course: sliced beef, pulled pork, smoked turkey and ham are all served lean but liberally, portion-wise. And for a bizarre barbecue twist there is grilled Cornish hen. But it's the killer ribs that make lunch and life worthwhile and will draw you back to the Shak.