Add cilantro, pickled jalapeños, and pickled jalapeño juice to a blender or food processor.
Blend until finely puréed, scraping down the sides as needed.
Add mayonnaise, sour cream, ranch seasoning, garlic powder, and salt to a medium bowl.
Stir until smooth.
Add the blended jalapeño mixture and lime juice. Stir until fully combined.
Add buttermilk, one tablespoon at a time, if you prefer a thinner consistency.
Cover and refrigerate for at least 4 hours.
Stir once more before serving, then adjust lime juice, salt, or jalapeño brine to taste.