Mix the beef, pork, breadcrumbs, onion, egg, milk, salt, allspice, nutmeg and pepper just until combined.
Shape the mixture into 24 meatballs and brown them in two batches in the oil, turning to color three sides, 6 minutes per batch.
Transfer the meatballs to a 6-quart slow cooker and add the broth and Worcestershire.
Cover and cook on low until the meatballs reach 160°F, 4 to 4 1/2 hours.
Whisk the cornstarch with the cold water, stir it into the gravy and cook on high until thickened, 8 minutes.
Turn off the cooker, let the bubbling stop and fold in the sour cream until smooth.
Boil the egg noodles until tender, drain them and toss them with the butter.
Serve the meatballs and gravy over the buttered noodles.