Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
Put the potato chunks in a saucepan and cover them with water. Bring to a boil, then simmer until the potatoes are tender when pierced with a fork, about 15 minutes.
While the potatoes cook, combine the ground beef, peas and carrots, breadcrumbs, egg, 1/4 cup milk, ketchup, Worcestershire sauce, salt, and pepper in a mixing bowl. Mix gently until just combined.
Divide the meat mixture among the prepared muffin cups. Press it in gently, leaving a slight dip in the center of each portion.
Bake until the meatloaf muffins are cooked through and their centers reach 160°F, about 20 to 25 minutes. Let them rest in the pan for 5 minutes.
Drain the potatoes well. Mash them with the butter, warm milk, and 1/4 teaspoon salt until smooth enough to pipe or spoon.
Carefully remove the meatloaf muffins from the pan. Spoon or pipe the mashed potatoes over each one before serving.