Special equipment: a pastry bag fitted with a large star tip; a kitchen torch
Preheat the oven to 375 degrees F. Spray a 12-cup standard muffin tin with cooking spray and set aside. Whisk together the eggs and 1 cup heavy cream in a large bowl and sprinkle with some salt and pepper. Add the leftover stuffing and thoroughly combine. Set aside for about 20 minutes to let the stuffing absorb the cream and eggs.
Divide the stuffing mixture among the muffin cups in the prepared pan, using an ice cream scoop for a rounded top. Bake in the middle of the oven until the tops are golden and they spring back to the touch, about 15 minutes.
Reheat the leftover mashed potatoes to very hot, then add to the bowl of a stand mixer fitted with a whisk attachment. Add the sour cream, melted butter and remaining 1/4 cup heavy cream. Whip on medium speed until light and fluffy, then season with salt and pepper. Transfer the potatoes to a pastry bag fitted with a large star tip.
Pipe about 1/4 cup potatoes on the tops of each stuffing cupcake. Torch the tops of the cupcakes so the edges of the potatoes get toasty and a little charred. Garnish with chives and serve.