Place a 15 x 10-inch baking pan in oven. Preheat oven to 425 degrees F. Meanwhile, microwave leftover potatoes in medium bowl 30 seconds on 100% power (high); stir to soften. Stir in the cheese, egg, flour, chives, and salt. Place panko in a shallow bowl. Using a small cookie scoop, drop potato mixture into panko, a few at a time. Roll to coat with panko and shape as desired. Add butter to preheated pan. Return to oven for 1 to 2 minutes or until melted. Tilt pan to distribute butter. Arrange potatoes in prepared pan.
Bake 8 minutes. Turn; bake 8 minutes more or until golden. For dipping sauce, combine sour cream and sriracha. Serve with potatoes.