Whisk the Base: In a large glass mixing bowl, combine 1 cup of heavy mayonnaise and 1 cup of whole buttermilk. Whisk vigorously until the mixture is completely smooth and free of any mayo clumps.
Add the Seasoning: Sprinkle the entire contents of the Hidden Valley Ranch packet over the liquid mixture.
Blend Thoroughly: Whisk the seasoning into the base until the herbs are evenly distributed throughout the liquid.
The Essential Chill Step: Cover the bowl tightly with plastic wrap or transfer the sauce to an airtight mason jar. Place it in the refrigerator for at least 30 minutes to 2 hours before serving. This step allows the dried herbs to rehydrate and infuse into the dairy.