Chop each cheese stick into half inch pieces (about 6-8 pieces for each cheese stick). Place on a plate and chill in the freezer until ready to use.
Add ground beef, bread crumbs, egg and egg yolk, garlic, onion powder, Italian seasoning and salt and pepper to a bowl. Mush the ingredients together with your hands until everything comes together.
Remove cheese from freezer. Scoop out about 4 tablespoons of meat mixture and roll into a ball. Gently push a piece of chilled mozzarella into the middle of the meatball. Roll the ball in your hands to cover the hole where the mozzarella cheese was pushed in. Place inside a greased slow cooker and repeat with remaining meat and cheese.
When all meatballs are in the slow cooker (*see note for alternate cooking method), cover and cook on high for 1-2 hours or on low 3-4 hours until meat it cooked through (no longer pink). The cheese may start melting out of the meatballs, that's okay.
If desired, sprinkle with a bit of fresh chopped parsley or dried Italian seasoning for garnish. Serve with your favorite marinara or alfredo sauce for dipping.