In a medium saucepan, combine the 2 quarts of water, Kosher salt, brown sugar, smashed garlic, black peppercorns, and fresh rosemary.
Place over medium heat and simmer just until the salt and sugar are completely dissolved (about 5 minutes). Remove the pan from the heat.
Immediately add the 2 quarts of ice cubes directly into the hot liquid to rapidly cool it down. This is crucial—if you mix hot water with buttermilk, the dairy will instantly curdle and separate!
Once the saltwater mixture is completely ice-cold, pour it into your large brining bucket or heavy-duty bag. Stir in the 1 gallon of buttermilk until well combined.
Submerge your fully thawed, raw turkey into the creamy liquid, breast-side down, ensuring it is entirely covered.
Transfer to the refrigerator and let the bird soak for 12 to 24 hours to achieve maximum tenderness.
Remove the turkey from the liquid. Gently wipe off the excess thick buttermilk mixture and pat the skin incredibly dry with paper towels before roasting to ensure a crispy finish.