Preheat the oven to 400°F. Spray 2 (24-count) mini muffin pans with cooking spray.

Place the sirloin in a large mixing bowl. Grate the onion and carrot into the bowl using the large holes of a box grater. Add the garlic, 1 teaspoon salt, ½ teaspoon pepper, breadcrumbs, egg, and Worcestershire sauce. Mix well to combine.

Using a small ice cream scoop, divide the mixture into the prepared pans and press down lightly. Top each meatloaf with about ½ teaspoon ketchup. Bake for 15 minutes or until the meatloaves are cooked through.

While the meatloaves are cooking, make the potatoes. Place the potatoes in a large pot of heavily salted, cold water. Bring to a boil and cook until the potatoes are tender, about 15 minutes. Drain the water from the pan and place the pan with the potatoes back on the heat. Cook, shaking the pan until they stop steaming, about 1-2 minutes. Place the potatoes in a mixer, preferably fitted with a paddle attachment. Add 1 teaspoon salt and beat until smooth. Add the butter and buttermilk and beat until creamy.

Pipe the potatoes on top of the meatloaves to look like frosting. (Can be made ahead up to this point and refrigerated, covered with plastic wrap. Reheat in a 300°F oven or microwave until warm.)

Before serving, remove the meatloaves from the pan and place them in mini cupcake liners.