In a large pot or container, combine the kosher salt, brown sugar, black peppercorns, thyme, rosemary, garlic, onion, and bay leaves.
Pour in the cold water and stir well until the salt and sugar dissolve completely.
Place the pork butt in the brine, ensuring it is fully submerged. If needed, add more water to cover the pork completely.
Cover the pot or container and refrigerate for at least 12 hours, but preferably 24 hours. This allows the flavors to penetrate the meat and helps to tenderize it. For even distribution of flavors, you can occasionally flip the pork butt while it's in the brine.
After brining, remove the pork butt from the brine and discard the liquid. Rinse the pork thoroughly under cold water to remove any excess salt from the surface.
At this point, you can proceed with your preferred cooking method, such as roasting or smoking the pork butt.