Preheat oven to 400 degrees. Liberally spray a non-stick mini muffin tin with non-stick spray.
Add cold, leftover mashed potatoes to a mixing bowl and heat for 1-2 minutes, just to get them soft enough to work with. They don't need to be hot at this point.
Add the flour, egg, and cheddar cheese to the potatoes and stir well to combine.
Scoop 1 ½ tablespoons of the potato mixture into each muffin well. A medium cookie scoop works well for this.
Bake for 25 minutes.
Let cool 15 minutes before running using a small spoon to loosen the edges and scoop the puffs from the muffin tin.
Serve with the ranch dip.
To make the ranch dip
While the potatoes are baking, whisk together the sour cream, milk, ranch, and chives. Set aside for serving.
We use leftover mashed potatoes from our Instant Pot Mashed Potatoes for this recipe. You can also purchase refrigerated mashed potatoes to use. You'll want a 24 ounce container.Season your mashed potatoes with chives or extra ranch seasoning, if you like.