Each tree will be a stack of 8 different size cookies. To assemble 3 trees, stack cookies on serving platter, starting with largest size cookie on bottom, and stacking each smaller size cookie on top, spreading frosting between each layer before stacking. Heat remaining frosting in microwavable bowl uncovered on High 10 to 15 seconds or until frosting can be stirred smooth. Drizzle over trees, and garnish with sprinkles. Sprinkle powdered sugar on platter to look like snow.