Arrange a rack in the middle of oven and heat to 400°F. Lightly coat the cups of a mini-muffin tin with cooking spray or butter.

Place 2 cups cooked mashed potatoes, 3 large beaten eggs, ¾ cup of the grated Parmesan or Gruyère cheese, ¼ cup minced fresh chives, and ¼ cup diced cooked bacon or ham if using in a large bowl and stir to combine. Season, if necessary, with kosher salt and black pepper.

Fill each muffin cup with the mashed potatoes. Sprinkle the tops with the remaining ¼ cup cheese.

Bake until the potato cups are set, browned on top, and heated through, about 20 minutes. Let cool for about 5 minutes in the pan, then use a spoon or knife to gently release them from the pan. Serve immediately with dollops of sour cream, if desired.