Mix the ground meats togehter (beef, pork, veal, and prosciutto).
Add the eggs.
Add the Parmigiano and Pecorino cheeses.
Add the Italian parsely and bread crumbs.
With your fingertips, gently mix everything to avoid over-mising or pressing down too hard (both will make the meatballs too hard and tough).
Form the mixture into small ½" to 1" sized meatballs.
Heat the butter in a saute' pan and fry the meatballs until well seared and deep brown, about 10 minutes, stirring to ensure all sides are cooked.
Prepare fresh meatballs or use frozen meatballs by baking them in the oven to re-heat (if frozen) or saute' fresh in butter.
Once reheated or cooked, add a thick, generous layer of meatballs over the cheesey layer.
Remove browned meatballs from the butter and place a plate. With an absorbent towel if desired, and then keep the meatballs warm until ready to use in a recipe.