Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, whisk the egg, milk, Worcestershire sauce, garlic powder, smoked paprika, salt, and black pepper. Stir in the breadcrumbs and chopped onion.
Add the ground beef. Mix gently with your hands or a fork just until combined. Avoid packing or overmixing the meat, which can make the muffins dense.
Divide the mixture evenly among the 8 prepared muffin cups. Press it in lightly, leaving the tops slightly rounded.
In a small bowl, stir together the barbecue sauce, ketchup, apple cider vinegar, and brown sugar. Spoon about half of the glaze over the meatloaf muffins.
Bake until the centers reach 160°F on an instant-read thermometer, about 20 to 25 minutes. Brush the remaining glaze over the tops during the last 5 minutes of baking.
Let the muffins rest in the pan for 5 minutes, then carefully lift them out and serve.