1. Cook the pasta: Bring a large pot of salted water to a rolling boil. Add your chosen pasta and cook according to package directions until al dente. This usually takes about 8-12 minutes. You want the pasta to be tender but still have a slight bite.

2. Drain and cool the pasta: Once cooked, drain the pasta thoroughly in a colander. Rinse it with cold water to stop the cooking process and cool it down quickly. This is crucial for a cold pasta salad, preventing the dressing from becoming oily. Let it drain well.

3. Prepare the chicken: While the pasta cooks, shred or dice your cooked chicken into bite-sized pieces. Aim for uniform pieces so they distribute evenly throughout the salad.

4. Whisk the dressing: In a large mixing bowl, combine the mayonnaise, buttermilk, and ranch seasoning mix. Whisk vigorously until the dressing is smooth and well combined. The ranch seasoning will dissolve into the liquids, creating a thick and creamy texture.

5. Combine ingredients: Add the cooled pasta, shredded chicken, chopped celery, chopped red onion, and thawed peas to the bowl with the dressing. Gently toss everything together until all ingredients are evenly coated with the creamy ranch dressing.

6. Season and chill: Taste the salad and add salt and black pepper as needed. Start with a pinch and add more if necessary. Cover the bowl and refrigerate the pasta salad for at least 30 minutes, or ideally 1-2 hours, to allow the flavors to meld and the salad to chill thoroughly.

7. Serve: Give the salad a final stir before serving. Garnish with fresh parsley or chives, if desired, for a pop of color and freshness.