Set the oven to 400 degrees F. Line a large rimmed baking sheet with parchment paper or foil, then lightly mist the lining with cooking spray.
In a large mixing bowl, combine the ground beef, ground pork, panko bread crumbs, grated onion, eggs, milk, salt, pepper, garlic powder, allspice, and 1/4 cup of the apple butter. Mix with clean hands just until combined; do not overmix.
Use a meatball scoop or tablespoon to portion the mixture into evenly sized balls, about 1 1/2 inches across. Roll gently between your palms until smooth and arrange in a single layer on the prepared baking sheet, leaving a little space between each one. You should end up with about 40 meatballs.
Bake for 15 minutes, until the meatballs are firm and lightly browned on the outside. They do not need to be fully cooked through at this stage.
While the meatballs bake, add the remaining 1 1/4 cups apple butter, barbecue sauce, apple cider vinegar, brown sugar, and cayenne pepper to a small saucepan. Warm over medium-low heat, stirring often, until smooth and slightly thinned, about 5 minutes. Taste and add extra cayenne if desired.
Remove the meatballs from the oven after the first bake. Pour the warm apple butter glaze evenly over the meatballs, making sure each one gets coated. Return the pan to the oven and bake for another 10 to 12 minutes, until the meatballs reach an internal temperature of 165 degrees F and the glaze is bubbly and slightly caramelized at the edges.
Let the meatballs sit on the pan for 2 to 3 minutes. Gently shake the pan to loosen the meatballs and coat them once more in any glaze pooled at the bottom. Transfer to a serving dish and serve warm with toothpicks for easy snacking.