Pat the beef cubes dry with paper towels and season generously with salt and pepper. Heat a large skillet over medium-high heat. Working in batches to avoid overcrowding, sear the beef on all sides until deeply browned, about 2-3 minutes per side. Transfer seared beef to the slow cooker.
Reduce skillet heat to medium. Add the beef broth, Worcestershire sauce, and tomato paste to the hot skillet. Use a wooden spoon to scrape up all the browned bits from the bottom of the pan. Let simmer for 1 minute, then pour this liquid over the beef in the slow cooker.
Add the cubed potatoes, minced garlic, dried thyme, and smoked paprika to the slow cooker with the beef. Stir gently to combine. Distribute the tablespoons of butter evenly over the top of the mixture.
Cover and cook on HIGH for 3.5 to 4 hours, or on LOW for 6-7 hours, until the beef is fork-tender and the potatoes are cooked through.
Once cooking is complete, if a thicker sauce is desired, create a slurry by mixing the cornstarch with 3 tablespoons of cold water until smooth. Stir the slurry into the slow cooker, cover, and cook on HIGH for an additional 15-20 minutes until the sauce has thickened. Stir in the chopped parsley.
Spoon the beef bites, potatoes, and plenty of garlic butter sauce onto round white plates. Garnish with additional fresh parsley and serve immediately.