Preheat your oven to 350°F. Line a muffin tin with cupcake liners or spray with non-stick cooking spray.
In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, onion, bell pepper, milk, egg, Worcestershire sauce, garlic powder, smoked paprika, salt, and black pepper. Mix until just combined; do not overmix.
Scoop the meat mixture into the prepared muffin tin, filling each cup about 3/4 full.
In a small bowl, mix together the barbecue sauce and bourbon (if using). Spoon a tablespoon of the sauce mixture over each meatloaf cupcake.
Bake in the preheated oven for 20-25 minutes, or until the meatloaf cupcakes are cooked through and reach an internal temperature of 160°F.
Remove from the oven and let cool for a few minutes. Garnish with chopped parsley before serving.