Preheat the oven to 350 F, grease a 9x13 inch pan with butter and dust with flour. Do not skip this step! You can alternatively use three 9 inch pans.
Fold in the carrot mixture and nuts. The nuts are optional, but I highly recommend adding them!
Gradually add in the powdered sugar and beat until smooth and fluffy. The frosting should hold a soft peak when you lift up the whisk/beater.
Source: Hot Chocolate Hits with permission to share her new cookbook. I did double the frosting.