Coarsely crush the black peppercorns. Reserve ½ tablespoon for finishing the sauce.
Add the chuck roast, butter, kosher salt, 1 tablespoon crushed peppercorns, rosemary, beef broth and ¼ cup cognac to the slow cooker.
Cover and cook on HIGH for 4 hours, stirring occasionally, until the beef is incredibly tender.
Whisk the cornstarch and cold water together until completely smooth.
Stir the slurry into the slow cooker. Cover and cook on HIGH for another 20 minutes, or until the sauce has thickened.
Stir in the heavy cream, Dijon mustard, Worcestershire sauce, remaining ½ tablespoon crushed peppercorns and remaining 2 tablespoons cognac.
Cook uncovered on HIGH for 5–10 minutes, stirring occasionally, until the sauce is creamy and the cognac has mellowed.
Taste and season with additional kosher salt as needed.
Spoon the steak bites and plenty of creamy cognac peppercorn sauce over Boursin Mashed Potatoes.
Finish with plenty of freshly cracked black pepper and serve immediately.