Adjust oven rack to upper middle position and preheat oven to 475 degrees. Coat rimmed baking sheet with vegetable oil.
Place hash browns and butter in a large microwave-safe bowl. Cover tightly with plastic wrap and microwave on high power until butter is melted and hash browns are defrosted, about 5 minutes. Remove plastic wrap, stir well, and let cool.
Whisk egg, scallions, cornstarch, salt, and pepper together in a medium bowl until thoroughly combined.
Add to hash brown mixture and stir until incorporated.
Divide mixture into 10 equal-sized balls and set aside on a large plate.
Place oiled baking sheet in oven and heat until oil is just beginning to smoke, about 6 minutes.
Remove baking sheet from oven, and working quickly, place potato balls on hot baking sheet.
Spray each ball with cooking spray then press each one into a 1/2-inch-thick disk using a metal spatula sprayed with non-stick cooking spray.
Bake until potato pancakes are golden brown on bottom, about 10 minutes. Flip pancakes over, press down using metal spatula, and bake until crisp and golden brown on second side, about 8 minutes.
Place pancakes on a plate lined with paper towels and let drain briefly. Garnish with sour cream and chives, if desired.