Preheat the oven to 350°F. In a large bowl, combine the ground ham, pork, graham cracker crumbs, milk, egg, and parsley. Mix together until evenly blended.
Season with salt and pepper to taste. Shape the mixture into 1-inch balls. Place them onto a greased baking sheet.
Bake for 25-30 minutes or until the internal temperature reaches 165°F. Meanwhile, prepare the sauce by combining ketchup, mustard, apple cider vinegar, and brown sugar in a medium bowl. Mix together until thoroughly blended.
Remove the ham balls from the oven and place them into the sauce. Stir to coat evenly.
Bake for another 10-15 minutes or until the ham balls are golden brown. Serve warm with extra sauce, if desired.