Mix dressing ingredients in a small bowl; keep chilled until ready to toss with salad. (Store in clean jar with lid.)
Bring 1 gallon of water and 2 Tbs. of salt to boil in a large soup kettle. Add pasta and, using package times as a guide, boil, stirring frequently and adding asparagus the last 1 minute, until just tender. Drain thoroughly (do not rinse) and dump onto a large, lipped cookie sheet. Set aside to cool while preparing remaining salad ingredients.
Place all salad ingredients (except buttermilk dressing) in a large bowl or transfer to a gallon-size zippered bag. (Can be covered and refrigerated several hours at this point.) When ready to serve, add dressing; toss to coat and serve.