Add potato cubes to a small saucepan and cover with water. Add 1/2 tsp salt and bring to a boil over medium-high heat. Cook for 10-15 minutes, or until the potatoes are tender enough to be pierced with a fork.
Preheat oven to 400 degrees. Line a small rimmed baking sheet with parchment paper and spray it with cooking spray.
At the end of cooking time for the potatoes, melt the butter in a small microwave-safe bowl. Warm the milk in a microwave-safe bowl. Set aside.
Drain the potatoes thoroughly and return to the saucepan over low heat for 2-3 minutes. This will help evaporate any excess moisture from potatoes.
Using a Potato ricer or masher, mash the potatoes until smooth and free of any lumps. Add 1 tablespoon of melted butter, warmed cream, salt, and pepper. Mix and add the egg yolk, mixing until combined.
Transfer the potatoes to a piping bag fitted with a large star tip. This tip will help create the swirls. Pipe the potatoes into 3-inch mounds on the prepared baking sheet. You will have 4 mounds of potatoes. They need to be about an inch apart.
Brush the potatoes with the remaining 1 tablespoon of melted butter.
Bake for 15 to 20 minutes or until potatoes are golden brown on edges and slightly crispy.
Remove from heat and garnish with chives. Serve immediately while hot.