Preheat oven to 375°F (190°C).

In a large bowl, combine ground beef, ground pork, finely chopped onion, eggs, pumpernickel breadcrumbs, and ground gingersnap cookie.

Add cider vinegar, prepared mustard, and salt to the meat mixture.

Mix well with a wooden spoon or your hands until all ingredients are evenly distributed.

Spoon the meatloaf mixture into an 8-inch square pan or a 9x5-inch loaf pan.

In a small bowl, combine brown sugar, cider vinegar, prepared mustard, and ketchup for the glaze.

Mix the glaze ingredients well.

Spoon the glaze evenly over the top of the meatloaf.

Bake in the preheated oven for approximately 55 minutes, or until the internal temperature reaches 160°F (71°C).

Let the meatloaf rest for a few minutes before slicing and serving.