Bring a large stockpot of generously salted water to a rolling boil. Add the rotini and cook according to package directions until al dente, usually 8 to 10 minutes.
Drain the pasta in a colander and rinse immediately under cold water to stop the cooking process. Shake off excess water and set aside to finish draining.
In a small bowl, whisk together the mayonnaise, sour cream, and buttermilk until smooth. Add the ranch seasoning packet and black pepper, whisking again until fully combined. Taste and adjust with salt if needed.
Refrigerate the dressing while you prepare the vegetables, letting the flavors meld.
Chop the broccoli into small, bite-sized pieces. Halve the cherry tomatoes and set them in a small bowl with a light pinch of salt for 10 minutes, then drain off any liquid.
Dice the cucumber and red onion into small, even pieces. Cook the bacon in a skillet over medium heat until crisp, about 8 minutes, then transfer to a paper towel-lined plate to cool before crumbling.
Add the drained, cooled pasta to a large glass mixing bowl. Add the broccoli, drained tomatoes, cucumber, and red onion on top of the pasta.
Pour the chilled ranch dressing over the pasta and vegetables. Use a rubber spatula to fold everything together gently until every piece of pasta is coated.
Fold in the shredded cheddar cheese and crumbled bacon, reserving a small amount of each to sprinkle on top for serving.
Cover the bowl tightly and refrigerate for at least 1 hour before serving. Just before serving, give the salad a final gentle stir and top with the reserved cheese, bacon, and chopped parsley.