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.

In a large bowl, combine cooked mashed potatoes, beaten eggs, 3/4 cup of the grated cheese, minced fresh chives, and diced cooked bacon or ham (if using). Stir to combine and season with kosher salt and black pepper as needed.

Fill each muffin cup with the mashed potato mixture and sprinkle the tops with the remaining 1/4 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.