Peel potatoes and remove the eyes. Cut each potato in half lengthwise and then cut into rough chunks, about 2 inches cubed. Transfer to a pot of cold water to soak for 5 minutes. Drain water and rinse the potatoes.
Add cold water about 1 inch above the potatoes. Put the lid on, leaving a small gap, and bring to a boil over medium-high heat. Once boiling, reduce heat to medium and cook the potatoes until tender, about 15 minutes.
Meanwhile, in a saucepan, heat the butter, heavy cream, and shio koji over medium-low heat. Turn off the heat and set aside to keep warm (you may need to reheat before adding to the potatoes).
When a skewer goes through the potato easily, turn off the heat. Drain the water completely. Rinse under hot running water and drain. Repeat 1-2 times to wash away excess starch. Drain really well.
Put the pot back on the stove and let them dry for a few minutes on low heat. With a potato masher, mash the potatoes until you have a creamy and even mash.
Pour the warm butter cream mixture over the mash and fold gently with a silicone spatula to combine. Season to taste with freshly ground black pepper. Transfer to a serving bowl. Using a spoon to create grooves and top with a slice of butter, chives, and more freshly ground black pepper. Enjoy!