Transfer potatoes to saucepan. Add cold water to cover and season with salt. Bring to a boil, then reduce heat to medium-low. Cover and simmer, stirring occasionally, for 15 to 20 minutes or until fork-tender. Drain well and transfer to bowl.

Mash potatoes until smooth. Add melted butter, sugar and salt. Sprinkle with cornstarch and mix well.

Divide potato mixture into 8 portions. Shape each portion into a ball. One at a time, flatten each ball into a patty and place a cube of mozzarella in the center. Shape potato mixture back into a ball to enclose mozzarella.

Transfer flour to small shallow bowl. Whisk egg in another small shallow bowl.

Dredge each potato cheese ball in flour, dip in egg and then coat in panko. Transfer to plate.

Pour enough oil into high-sided skillet to reach 1 inch up sides of pan. Heat over medium heat until shimmering or instant-read thermometer registers 350°F. Fry potato cheese balls, in batches if needed, for 5 minutes or until golden brown all over. Transfer to paper towel to drain.

Let cool for 5 minutes and garnish with chives to serve.