Place cooling rack on cookie sheet or waxed paper to catch icing drips. Cut cake into 9 rows by 6 rows. Working with 6 pieces at a time, remove cake pieces from pan and place on cooling rack. Spoon icing evenly over top and sides of cake pieces, letting icing coat sides. (Icing that drips off can be reused.) Let stand until icing is set, about 2 hours.