Crumble cooled cake into a large mixing bowl: Cut a baked 13 x 9-inch cake into four equal sections. Remove a section from the pan, break it in half and rub the two pieces together over a large bowl, making sure to crumble any large pieces that fall off. You can also use a fork to break apart any larger pieces. Repeat with each section until entire cake is crumbled into a fine texture. (If large pieces are mixed in, the cake balls may turn out lumpy and bumpy.) You should not see any large pieces of cake.